<PAGE>

                                                      Registration No. 333-


                                 UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C.  20549

                                 FORM S-8

            REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933

                        CSG SYSTEMS INTERNATIONAL, INC
        --------------------------------------------------------------
            (Exact name of registrant as specified in its charter)

            Delaware                                47-0783182
--------------------------------------------------------------------------------
(State or other jurisdiction of         (I.R.S. Employer Identification No.)
incorporation or organization)

           7887 East Belleview Ave., Suite 1000, Englewood, CO 80111
        --------------------------------------------------------------
           (Address of Principal Executive Offices)        (Zip Code)


                        CSG Systems International, Inc.
                 Stock Option Plan for Non-Employee Directors
        --------------------------------------------------------------
                           (Full title of the plan)


                       Joseph T. Ruble, General Counsel
                        CSG Systems International, Inc.
           7887 East Belleview Ave., Suite 1000, Englewood, CO 80111
        --------------------------------------------------------------
                    (Name and address of agent for service)


                                (303) 796-2850
        --------------------------------------------------------------
         (Telephone number, including area code, of agent for service)

                                   Copy to:
                               Howard J. Kaslow
                        8712 West Dodge Road, Suite 300
                             Omaha, NE  68114-3419

                        Calculation of Registration Fee

<TABLE>
<CAPTION>
                                                     Proposed maximum    Proposed maximum
Title of securities                   Amount to be    offering price    aggregate offering       Amount of
to be registered                       registered        per unit              price         registration  fee
-----------------------------------  --------------  -----------------  -------------------  -----------------
<S>                                  <C>             <C>                <C>                  <C>
Common Stock, par                    250,000 shares      $50.85  (1)        $12,712,500  (2)     $3,356.10
value $0.01 per share
</TABLE>

(1)  Estimated solely for purposes of calculating the registration fee.  The
     price is based upon the average of the high and low prices of CSG Systems
     International, Inc. Common Stock on July 21, 2000, as reported on the
     National Association of Securities Dealers Automated Quotation system.

(2)  The aggregate offering price is based upon the proposed maximum offering
     price per unit.
<PAGE>

                                    PART II

              INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Incorporation of Documents by Reference
---------------------------------------

      The documents listed in (a) through (c) below are incorporated by
reference in this registration statement; and all documents subsequently filed
by the registrant pursuant to Sections 13(a), 13(c), 14, and 15(d) of the
Securities Exchange Act of 1934, as amended (the "Exchange Act"), prior to the
filing of a post-effective amendment which indicates that all securities offered
have been sold or which deregisters all securities then remaining unsold, shall
be deemed to be incorporated by reference in this registration statement and to
be part thereof from the date of filing of such documents.

      (a)  The registrant's latest annual report filed pursuant to Section 13(a)
or 15(d) of the Exchange Act.

      (b)  All other reports filed by the registrant pursuant to Section 13(a)
or 15(d) of the Exchange Act since the end of the fiscal year covered by the
registrant document referred to in (a) above.

      (c)  The description of the class of securities being offered contained in
a registration statement filed under the Exchange Act, including any amendment
or report filed for the purpose of updating such description.

Indemnification of Directors and Officers.
-----------------------------------------

      Section 145 of the General Corporation Law of the State of Delaware
permits indemnification by a corporation of certain officers, directors,
employees, and agents.  Consistent therewith, the registrant's By-Laws require
the registrant, to the maximum extent and in the manner permitted by the
Delaware General Corporation Law, to indemnify each of its directors and
officers against expenses (including attorneys' fees), judgments, fines, and
amounts paid in settlement actually and reasonably incurred in connection with
any proceeding to which such director or officer was, is, or may be a party,
arising by reason of the fact that such person is or was a director, officer,
employee, or agent of the registrant.  For purposes of this provision, a
"director" or "officer" of the registrant includes any person (i) who is or was
a director or officer of the registrant, (ii) who is or was serving at the
request of the registrant as a director or officer of another corporation,
partnership, joint venture, trust, or other enterprise, or (iii) who was a
director or officer of a corporation that was a predecessor corporation of the
registrant or of another enterprise at the request of such predecessor
corporation.  The registrant may, to the extent and in the manner permitted by
the General Corporation Law of Delaware, indemnify each of its employees and
agents (other than directors and officers) against expenses (including
attorneys' fees), judgments, fines, and amounts paid in settlement actually and
reasonably incurred in connection with any proceeding to which such person was,
is, or may be a party arising by reason of the fact that such person is or was
an employee or agent of the registrant.  For purposes of this provision, an
"employee" or "agent" of the registrant (other than a director or officer)
includes any person (i) who is or was an employee or agent of the registrant,
(ii) who is or was serving at the request of the registrant as an employee or
agent of another corporation, partnership, joint venture, trust, or

                                       2
<PAGE>

other enterprise, or (iii) who was an employee or agent of a corporation which
was a predecessor corporation of the registrant or of another enterprise at the
request of such predecessor corporation.

      The registrant maintains directors and officers liability insurance for
the benefit of its directors and officers.

      The registrant has entered into separate indemnification agreements with
each of its directors and certain officers pursuant to which the registrant has
agreed, among other things, and subject to certain limited exceptions (i) to
indemnify them to the fullest extent permitted by law against any claims and
expenses incurred in connection therewith arising out of any event or occurrence
relating to their status as director, officer, employee, agent, or fiduciary of
the Company or of any other entity as to which they served at the request of the
registrant or by reason of any action or inaction while serving in such
capacity, and (ii) to advance any such expenses no later than five days after
demand.

      Under a registration rights agreement between the registrant and certain
of its stockholders, the registrant has agreed to indemnify each stockholder
selling his, her, or its shares thereunder in connection with any expenses,
losses, claims, damages, or liabilities arising out of certain acts or omissions
of the registrant.

Undertakings
------------

      (a)   The undersigned registrant hereby undertakes:

      1.  To file, during any period in which offers or sales are being made, a
post-effective amendment to this registration statement:

      (i)   To include any prospectus required by Section 10(a)(3) of the
      Act;

      (ii)  To reflect in the prospectus any facts or events arising after
      the effective date of this registration statement (or the most recent
      post-effective amendment thereof) which, individually or in the aggregate,
      represent a fundamental change in the information set forth in this
      registration statement;

      (iii) To include any material information with respect to the plan of
      distribution not previously disclosed in this registration statement or
      any material change to such information in this registration statement;

provided, however, that paragraphs (a)(1)(i) and (a)(1)(ii) do not apply if the
information required to be included in a post-effective amendment by those
paragraphs is contained in periodic reports filed by the registrant pursuant to
Section 13 or Section 15(d) of the Exchange Act that are incorporated by
reference in this registration statement.

      2.  That, for the purpose of determining any liability under the
Securities Act of 1933, as amended (the "Act"), each such post-effective
amendment shall be deemed to be a new registration statement relating to the
securities offered therein, and the offering of such securities at that time
shall be deemed to be the initial bona fide offering thereof.

                                       3
<PAGE>

      3.  To remove from registration by means of a post-effective amendment any
of the securities being registered which remain unsold at the termination of the
offering.

          (b)  The undersigned registrant hereby undertakes that, for purposes
of determining any liability under the Act, each filing of the registrant's
annual report pursuant to Section 13(a) or Section 15(d) of the Exchange Act
(and, where applicable, each filing of an employee benefit plan's annual report
pursuant to Section 15(d) of the Exchange Act) that is incorporated by reference
in this registration statement shall be deemed to be a new registration
statement relating to the securities offered therein, and the offering of such
securities at that time shall be deemed to be the initial bona fide offering
thereof.

          (c)  Insofar as indemnification for liabilities arising under the Act
may be permitted to directors, officers, and controlling persons of the
registrant pursuant to the foregoing provisions, or otherwise, the registrant
has been advised that in the opinion of the Securities and Exchange Commission
such indemnification is against public policy as expressed in the Act and is,
therefore, unenforceable.  In the event that a claim for indemnification against
such liabilities (other than the payment by the registrant of expenses incurred
or paid by a director, officer, or controlling person of the registrant in the
successful defense of any action, suit, or proceeding) is asserted by such
director, officer, or controlling person in connection with the securities being
registered, the registrant will, unless in the opinion of counsel the matter has
been settled by controlling precedent, submit to a court of appropriate
jurisdiction the question whether such indemnification by it is against public
policy as expressed in the Act and will be governed by the final adjudication of
such issue.

                                       4
<PAGE>

                                  SIGNATURES

      Pursuant to the requirements of the Securities Act of 1933, the Registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-8 and has duly caused this Registration
Statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Englewood, State of Colorado, on July 24, 2000.

                                  CSG SYSTEMS INTERNATIONAL, INC.

                                  By:  /s/ Neal C. Hansen
                                      ----------------------------------
                                      Neal C. Hansen, Chairman of
                                      the Board and Chief Executive
                                      Officer (Principal Executive
                                      Officer)

                               POWER OF ATTORNEY

      We, the undersigned officers and directors of CSG Systems International,
Inc., hereby severally and individually constitute and appoint Neal C. Hansen,
John P. Pogge, and Greg Parker, and each of them individually, the true and
lawful attorneys and agents of each of us to execute in the name, place and
stead of each of us (individually and in any capacity stated below) any and all
amendments to this Registration Statement on Form S-8, and all instruments
necessary or advisable in connection therewith, and to file the same with the
Securities and Exchange Commission, each of said attorneys and agents to have
power to act with or without the others and to have full power and authority to
do and perform in the name and on behalf of each of the undersigned every act
whatsoever necessary or advisable to be done in the premises as fully and to all
intents and purposes as any of the undersigned might or could do in person, and
we hereby ratify and confirm our signatures as they may be signed by our said
attorneys and agents and each of them individually to any and all such
amendments and other instruments.

      Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities and on the date indicated.

<TABLE>
<CAPTION>
     Name                                Title                                             Date
     ----                                -----                                             ----
<S>                                 <C>                                               <C>
/s/ Neal C. Hansen                  Chairman of the Board, Chief Executive Officer    July 24, 2000
------------------                  and Director (Principal Executive Officer)
    Neal C. Hansen

/s/ Greg Parker                     Chief Financial Officer                           July 24, 2000
---------------
    Greg Parker

/s/ Randy Wiese                     Controller (Principal Accounting Officer)         July 24, 2000
---------------
    Randy Wiese

/s/ George F. Haddix                Director                                          July 24, 2000
--------------------
    George F. Haddix

/s/ John P. Pogge                   Director                                          July 24, 2000
-----------------
    John P. Pogge

---------------------               Director                                          -------, 2000
    Royce Holland

/s/ Janice Obuchowski               Director                                          July 24, 2000
---------------------
    Janice Obuchowski

/s/ Bernard W. Reznicek             Director                                          July 24, 2000
-----------------------
    Bernard W. Reznicek

/s/ Rockwell A. Schnabel            Director                                          July 24, 2000
------------------------
    Rockwell A. Schnabel

/s/ Frank V. Sica                   Director                                          July 24, 2000
-----------------
    Frank V. Sica
</TABLE>
<PAGE>

                                 EXHIBIT INDEX



          Exhibit No.                        Description
          -----------                        -----------

             5                          Opinion of Legal Counsel
            23                          Consent of Arthur Andersen LLP
[{"type": "S-8", "sequence": "1", "filename": "0001.txt", "description": "FORM S-8 STOCK OPTION PLAN FOR NON-EMPLOYEE DIR.", "secsgml_size_bytes": 15893}]
{"company-data": {"conformed-name": "CSG SYSTEMS INTERNATIONAL INC", "cik": "0001005757", "assigned-sic": "7374", "irs-number": "470783182", "state-of-incorporation": "DE", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-42202", "film-number": "678638"}, "business-address": {"street1": "7887 EAST BELLEVIEW AVE", "street2": "SUITE 1000", "city": "ENGLEWOOD", "state": "CO", "zip": "80111", "phone": "3037962850"}, "mail-address": {"street1": "5251 DTC PARKWAY SUITE 625", "city": "ENGLEWOOD", "state": "CO", "zip": "80111"}}
Registration No. 333-

                                 UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C.  20549

                                 FORM S-8

            REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933

                        CSG SYSTEMS INTERNATIONAL, INC
        --------------------------------------------------------------
            (Exact name of registrant as specified in its charter)

            Delaware                                47-0783182
--------------------------------------------------------------------------------
(State or other jurisdiction of         (I.R.S. Employer Identification No.)
incorporation or organization)

           7887 East Belleview Ave., Suite 1000, Englewood, CO 80111
        --------------------------------------------------------------
           (Address of Principal Executive Offices)        (Zip Code)

                        CSG Systems International, Inc.
                 Stock Option Plan for Non-Employee Directors
        --------------------------------------------------------------
                           (Full title of the plan)

                       Joseph T. Ruble, General Counsel
                        CSG Systems International, Inc.
           7887 East Belleview Ave., Suite 1000, Englewood, CO 80111
        --------------------------------------------------------------
                    (Name and address of agent for service)

                                (303) 796-2850
        --------------------------------------------------------------
         (Telephone number, including area code, of agent for service)

                                   Copy to:
                               Howard J. Kaslow
                        8712 West Dodge Road, Suite 300
                             Omaha, NE  68114-3419

                        Calculation of Registration Fee
Proposed maximum    Proposed maximum
Title of securities                   Amount to be    offering price    aggregate offering       Amount of
to be registered                       registered        per unit              price         registration  fee
-----------------------------------  --------------  -----------------  -------------------  -----------------
Common Stock, par                    250,000 shares      $50.85  (1)        $12,712,500  (2)     $3,356.10
value $0.01 per share
(1)  Estimated solely for purposes of calculating the registration fee.  The
     price is based upon the average of the high and low prices of CSG Systems
     International, Inc. Common Stock on July 21, 2000, as reported on the
     National Association of Securities Dealers Automated Quotation system.

(2)  The aggregate offering price is based upon the proposed maximum offering
     price per unit.
PART II

              INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Incorporation of Documents by Reference
---------------------------------------

      The documents listed in (a) through (c) below are incorporated by
reference in this registration statement; and all documents subsequently filed
by the registrant pursuant to Sections 13(a), 13(c), 14, and 15(d) of the
Securities Exchange Act of 1934, as amended (the "Exchange Act"), prior to the
filing of a post-effective amendment which indicates that all securities offered
have been sold or which deregisters all securities then remaining unsold, shall
be deemed to be incorporated by reference in this registration statement and to
be part thereof from the date of filing of such documents.

      (a)  The registrant's latest annual report filed pursuant to Section 13(a)
or 15(d) of the Exchange Act.

      (b)  All other reports filed by the registrant pursuant to Section 13(a)
or 15(d) of the Exchange Act since the end of the fiscal year covered by the
registrant document referred to in (a) above.

      (c)  The description of the class of securities being offered contained in
a registration statement filed under the Exchange Act, including any amendment
or report filed for the purpose of updating such description.

Indemnification of Directors and Officers.
-----------------------------------------

      Section 145 of the General Corporation Law of the State of Delaware
permits indemnification by a corporation of certain officers, directors,
employees, and agents.  Consistent therewith, the registrant's By-Laws require
the registrant, to the maximum extent and in the manner permitted by the
Delaware General Corporation Law, to indemnify each of its directors and
officers against expenses (including attorneys' fees), judgments, fines, and
amounts paid in settlement actually and reasonably incurred in connection with
any proceeding to which such director or officer was, is, or may be a party,
arising by reason of the fact that such person is or was a director, officer,
employee, or agent of the registrant.  For purposes of this provision, a
"director" or "officer" of the registrant includes any person (i) who is or was
a director or officer of the registrant, (ii) who is or was serving at the
request of the registrant as a director or officer of another corporation,
partnership, joint venture, trust, or other enterprise, or (iii) who was a
director or officer of a corporation that was a predecessor corporation of the
registrant or of another enterprise at the request of such predecessor
corporation.  The registrant may, to the extent and in the manner permitted by
the General Corporation Law of Delaware, indemnify each of its employees and
agents (other than directors and officers) against expenses (including
attorneys' fees), judgments, fines, and amounts paid in settlement actually and
reasonably incurred in connection with any proceeding to which such person was,
is, or may be a party arising by reason of the fact that such person is or was
an employee or agent of the registrant.  For purposes of this provision, an
"employee" or "agent" of the registrant (other than a director or officer)
includes any person (i) who is or was an employee or agent of the registrant,
(ii) who is or was serving at the request of the registrant as an employee or
agent of another corporation, partnership, joint venture, trust, or

                                       2
other enterprise, or (iii) who was an employee or agent of a corporation which
was a predecessor corporation of the registrant or of another enterprise at the
request of such predecessor corporation.

      The registrant maintains directors and officers liability insurance for
the benefit of its directors and officers.

      The registrant has entered into separate indemnification agreements with
each of its directors and certain officers pursuant to which the registrant has
agreed, among other things, and subject to certain limited exceptions (i) to
indemnify them to the fullest extent permitted by law against any claims and
expenses incurred in connection therewith arising out of any event or occurrence
relating to their status as director, officer, employee, agent, or fiduciary of
the Company or of any other entity as to which they served at the request of the
registrant or by reason of any action or inaction while serving in such
capacity, and (ii) to advance any such expenses no later than five days after
demand.

      Under a registration rights agreement between the registrant and certain
of its stockholders, the registrant has agreed to indemnify each stockholder
selling his, her, or its shares thereunder in connection with any expenses,
losses, claims, damages, or liabilities arising out of certain acts or omissions
of the registrant.

Undertakings
------------

      (a)   The undersigned registrant hereby undertakes:

      1.  To file, during any period in which offers or sales are being made, a
post-effective amendment to this registration statement:

      (i)   To include any prospectus required by Section 10(a)(3) of the
      Act;

      (ii)  To reflect in the prospectus any facts or events arising after
      the effective date of this registration statement (or the most recent
      post-effective amendment thereof) which, individually or in the aggregate,
      represent a fundamental change in the information set forth in this
      registration statement;

      (iii) To include any material information with respect to the plan of
      distribution not previously disclosed in this registration statement or
      any material change to such information in this registration statement;

provided, however, that paragraphs (a)(1)(i) and (a)(1)(ii) do not apply if the
information required to be included in a post-effective amendment by those
paragraphs is contained in periodic reports filed by the registrant pursuant to
Section 13 or Section 15(d) of the Exchange Act that are incorporated by
reference in this registration statement.

      2.  That, for the purpose of determining any liability under the
Securities Act of 1933, as amended (the "Act"), each such post-effective
amendment shall be deemed to be a new registration statement relating to the
securities offered therein, and the offering of such securities at that time
shall be deemed to be the initial bona fide offering thereof.

                                       3
3.  To remove from registration by means of a post-effective amendment any
of the securities being registered which remain unsold at the termination of the
offering.

          (b)  The undersigned registrant hereby undertakes that, for purposes
of determining any liability under the Act, each filing of the registrant's
annual report pursuant to Section 13(a) or Section 15(d) of the Exchange Act
(and, where applicable, each filing of an employee benefit plan's annual report
pursuant to Section 15(d) of the Exchange Act) that is incorporated by reference
in this registration statement shall be deemed to be a new registration
statement relating to the securities offered therein, and the offering of such
securities at that time shall be deemed to be the initial bona fide offering
thereof.

          (c)  Insofar as indemnification for liabilities arising under the Act
may be permitted to directors, officers, and controlling persons of the
registrant pursuant to the foregoing provisions, or otherwise, the registrant
has been advised that in the opinion of the Securities and Exchange Commission
such indemnification is against public policy as expressed in the Act and is,
therefore, unenforceable.  In the event that a claim for indemnification against
such liabilities (other than the payment by the registrant of expenses incurred
or paid by a director, officer, or controlling person of the registrant in the
successful defense of any action, suit, or proceeding) is asserted by such
director, officer, or controlling person in connection with the securities being
registered, the registrant will, unless in the opinion of counsel the matter has
been settled by controlling precedent, submit to a court of appropriate
jurisdiction the question whether such indemnification by it is against public
policy as expressed in the Act and will be governed by the final adjudication of
such issue.

                                       4
SIGNATURES

      Pursuant to the requirements of the Securities Act of 1933, the Registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-8 and has duly caused this Registration
Statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Englewood, State of Colorado, on July 24, 2000.

                                  CSG SYSTEMS INTERNATIONAL, INC.

                                  By:  /s/ Neal C. Hansen
                                      ----------------------------------
                                      Neal C. Hansen, Chairman of
                                      the Board and Chief Executive
                                      Officer (Principal Executive
                                      Officer)

                               POWER OF ATTORNEY

      We, the undersigned officers and directors of CSG Systems International,
Inc., hereby severally and individually constitute and appoint Neal C. Hansen,
John P. Pogge, and Greg Parker, and each of them individually, the true and
lawful attorneys and agents of each of us to execute in the name, place and
stead of each of us (individually and in any capacity stated below) any and all
amendments to this Registration Statement on Form S-8, and all instruments
necessary or advisable in connection therewith, and to file the same with the
Securities and Exchange Commission, each of said attorneys and agents to have
power to act with or without the others and to have full power and authority to
do and perform in the name and on behalf of each of the undersigned every act
whatsoever necessary or advisable to be done in the premises as fully and to all
intents and purposes as any of the undersigned might or could do in person, and
we hereby ratify and confirm our signatures as they may be signed by our said
attorneys and agents and each of them individually to any and all such
amendments and other instruments.

      Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities and on the date indicated.
Name                                Title                                             Date
     ----                                -----                                             ----
/s/ Neal C. Hansen                  Chairman of the Board, Chief Executive Officer    July 24, 2000
------------------                  and Director (Principal Executive Officer)
    Neal C. Hansen

/s/ Greg Parker                     Chief Financial Officer                           July 24, 2000
---------------
    Greg Parker

/s/ Randy Wiese                     Controller (Principal Accounting Officer)         July 24, 2000
---------------
    Randy Wiese

/s/ George F. Haddix                Director                                          July 24, 2000
--------------------
    George F. Haddix

/s/ John P. Pogge                   Director                                          July 24, 2000
-----------------
    John P. Pogge

---------------------               Director                                          -------, 2000
    Royce Holland

/s/ Janice Obuchowski               Director                                          July 24, 2000
---------------------
    Janice Obuchowski

/s/ Bernard W. Reznicek             Director                                          July 24, 2000
-----------------------
    Bernard W. Reznicek

/s/ Rockwell A. Schnabel            Director                                          July 24, 2000
------------------------
    Rockwell A. Schnabel

/s/ Frank V. Sica                   Director                                          July 24, 2000
-----------------
    Frank V. Sica
EXHIBIT INDEX

          Exhibit No.                        Description
          -----------                        -----------

             5                          Opinion of Legal Counsel
            23                          Consent of Arthur Andersen LLP