@staticmethod
def _get_msisdn_phonenumber(phonenumber):
    """
        convert the phone number to something more msisdn compliant

        from http://www.msisdn.org/:
          In GSM standard 1800, this number is built up as
            MSISDN = CC + NDC + SN
            CC = Country Code
            NDC = National Destination Code
            SN = Subscriber Number

        there are two version of the msisdn: the global definition and
        the local definition, with the difference, that the global definition
        might start with an +CC country code. in this conversion routine, the
        global prefixing is ignored
        """
    msisdn = []
    prefix = False
    if phonenumber.strip()[0] == '+':
        prefix = True
    for character in phonenumber:
        if character.isdigit():
            msisdn.append(character)
    phone = ''.join(msisdn)
    if prefix:
        return '+' + phone
    else:
        return phone
