code: def credentials(self):
          
          if self._credentials is None:
              self._credentials = CredentialList(
                  self._version,
                  account_sid=self._solution['account_sid'],
                  credential_list_sid=self._solution['sid'],
              )
          return self._credentials
docs: Access the credentials
  
          :returns: twilio.rest.api.v2010.account.sip.credential_list.credential.CredentialList
          :rtype: twilio.rest.api.v2010.account.sip.credential_list.credential.CredentialList