Can't send messages with T mobile Iphone 4s?

So I have an Iphone 4s that is actually locked to AT&T. I tried to unlock it but with no success. So I bought a Gevey Ultra S Sim that can read any carrier's Sim Card and it worked perfectly the first day. But now I'm not being able to send messages although i can receive them. I really need help with this. Thanks
firmware 2.0.12
version 5.1.1
Also someone suggested me to create an AVN. I don't even know what that is.

You can't send mesages because tmobile doesn't carry iphones. Iphones are only avaible through sprint, verizon, at&t

Your best option without problems is to jailbreak it and unlock the phone that way

here's a link to a couple jailbreak programs (i personally prefer redsn0w)
http://www.jailbreaknation.com/downloads1/download

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