Iphone 4s unlocked with tmobile imessage problems?

Hi, I have an Iphone 4s unlocked with T-mobile. Imessage was working perfectly until tonight. It keeps saying waiting for activation. I turned it off and on then suddenly a message popped up saying "Your carrier may charge for sms messaging" something like that. Now imessage does not work. I have reset all settings, but it did not do anything. My iphone is not jailbroken. Is there any way i can fix it? Will it be fixed at all is my main question.

First reset the phone to its original state and if that doesn't work I'd cheek YouTube, and if that doesn't work take it back to the store

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