I also had the same problem a few days ago, but managed to input my mobile number for flight reminder by SMS. Firstly, make sure the Home and Business fields are blank. In the Mobile field, input your mobile number in the following format: CountryCode + AreaCode + Number (No + sign or 00's at the beginning or spaces) e.g. if your number is +44 798 5155014, then enter "447985155014" and it should accept it. If it still doesn't, try again later.