My Samsung Galaxy S2 (T-Mobile) won't let me send text messages.. I can receive them but I can't respond back. Also, I can send MMS but not SMS. Whenever I send an SMS, it just keeps loading and says "sending" then finally says 'failed' w/ a red ! sign beside it. I can also call people and receive calls but whenever somebody in my contacts call me, it always say "Unknown number". I already checked my message centre number and it has the right number in it so I don't think that's the issue. I thought it was because I just upgraded to jellybean 4.1.2 but then I downgraded to ICS and it's still like that.... help?