private static function clean($msisdn)
    {
        $msisdn = preg_replace("/[^0-9]/", "", $msisdn);

        // We remove the 0 or 63 from the number
        if (substr($msisdn, 0, 1) == '0') {
            $msisdn = substr($msisdn, 1, strlen($msisdn));
        } else if (substr($msisdn, 0, 2) == '63') {
            $msisdn = substr($msisdn, 2, strlen($msisdn));
        }

        return $msisdn;
    }