def METHOD_NAME(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