code: def credential_lists(self):
          
          if self._credential_lists is None:
              self._credential_lists = CredentialListList(
                  self._version,
                  account_sid=self._solution['account_sid'],
              )
          return self._credential_lists
docs: Access the credential_lists
  
          :returns: twilio.rest.api.v2010.account.sip.credential_list.CredentialListList
          :rtype: twilio.rest.api.v2010.account.sip.credential_list.CredentialListList