docs: def e_164(msisdn: str) -> str:
      """
      Returns the msisdn in E.164 international format.
      """
      # Phonenumbers library requires the + to identify the country, so we add it if it
      # does not already exist
      number = phonenumbers.parse("+{}".format(msisdn.lstrip("+")), None)
      return phonenumbers.format_number(number, phonenumbers.PhoneNumberFormat.E164)