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

[kazakh]

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.

 

[kazakh]

any ideas guys?

 

[kevroc]

Have you tried this?
http://support.t-mobile.com/docs/DOC-2383

 

[kazakh]

i have. didn't work.

I just went to a T-mobile store and asked them to give me a new sim card, which fixed the problem. So i assume the problem was in the sim card itself, which is weird because i have a second line with t-mobile and the sim card there is doing the same thing