My Iphone5 won't let me send normal text messages? (tmobile)?

I just got the Iphone 5 in tmobile. And I can send imessages, but I can't send text messages. I can receive them, but it won't let me reply. It says "not delivered". Help. I already looked at settings and it says that the mms or what not is on, so I'm not really sure of the problem.

Go to settings, messages and make sure that your send as SMS is on, so if iMessage is unavailable it will send as SMS

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