old_contents: from django import forms
  from django.utils.translation import ugettext as _
  from django.core.exceptions import ValidationError
  from phonenumber_field.formfields import PhoneNumberField
  
  
  class CallbackForm(forms.Form):
      name = forms.CharField(
          label=_('Имя'),
          max_length=127,
          widget=forms.TextInput(attrs={'placeholder': _('Владимир')}),
          required=False,
      )
      phone_number = PhoneNumberField(
          label=_('Номер телефона'),
          widget=forms.TextInput(attrs={'placeholder': '+79995475707'}),
      )
      extra_info = forms.CharField(widget = forms.HiddenInput(), required = False)
      def is_valid(self):
          valid = super().is_valid()
  
          # It is necessary for inter russian calls
          # It is commonplace to call by 8 (983)... against +7 (983)...
          raw_data = self.data.get('phone_number')
          if not self.cleaned_data.get('phone_number') and raw_data and raw_data[0] == '8':
              self.cleaned_data['phone_number'] = "+7" + raw_data[1:]
              if CallbackForm(self.cleaned_data).is_valid():
                  self._errors.pop('phone_number', None)
                  return True
          return valid
new_contents: from django import forms
  from django.utils.translation import ugettext as _
  from django.core.exceptions import ValidationError
  from phonenumber_field.formfields import PhoneNumberField
  
  
  class RussianPhoneNumberFormMixin(object):
      PHONE_NUMBER_FIELD = 'phone_number'
      def is_valid(self, *args, **kwargs):
          valid = super(RussianPhoneNumberFormMixin, self).is_valid()
  
          # It is necessary for inter russian calls
          raw_data = self.data.get(self.PHONE_NUMBER_FIELD)
          if not self.cleaned_data.get(self.PHONE_NUMBER_FIELD):
              transformed8 = transform_russian8_phone_number(raw_data)
              if transformed8!=raw_data:
                  self.cleaned_data[self.PHONE_NUMBER_FIELD] = transformed8
                  self._errors.pop(self.PHONE_NUMBER_FIELD, None)
                  valid = super(RussianPhoneNumberFormMixin, self).is_valid()
          return valid
  
  
  
  class CallbackForm(RussianPhoneNumberFormMixin, forms.Form):
      name = forms.CharField(
          label=_('Имя'),
          max_length=127,
          widget=forms.TextInput(attrs={'placeholder': _('Владимир')}),
          required=False,
      )
      phone_number = PhoneNumberField(
          label=_('Номер телефона'),
          widget=forms.TextInput(attrs={'placeholder': '+79995475707'}),
      )
      extra_info = forms.CharField(widget = forms.HiddenInput(), required = False)
  
  
  def transform_russian8_phone_number(phone_number):
      # It is commonplace to call by 8 (983)... against +7 (983)...
      if phone_number and phone_number[0]=='8':
          phone_number = '+7' + phone_number[1:]
      return phone_number
subject: Put russian phone number convertion from applications.forms to own function and mixin for using in another apps.