Have you tried entering a DIFFERENT phone number to see if that works? It may be that your phone number is also associated with another MP account. This could be for someone in your household, someone who used to have your phone number, someone who entered their own phone number wrong, a duplicate account in your name that was opened automatically by a credit card or other service and which you don't even know about (this happened to me), or something else. If more than one account is associated with a phone number then the system doesn't recognize you.