switched to T-mobile. ATT iphone 4 unlocked doesn't show "my number"

Well, i just switched from ATT to T-mobile and am using my ATT iphone 4 which was unlocked thru ATT. Every time I try to open messages, it says the following: "MMS messaging requires your phone number. Press 'Settings" to go to the phone settings and enter your phone number'". When I do so and try to enter my phone number, it says that the phone number cannot be saved.

Tried taking out/putting back in the sim card, connecting it to itunes, resetting networks settings - nothing works.

Any idea what that could be? I assume that's a more or less common problem given how many questions show up on Google - but nobody seems to have a legit answer.

thanks.