IPhone 4 won't let me send text?

I got an unlocked t mobile iPhone like two or three weeks ago.recently, i had to change out my Sim card.ever since then my iPhone won't let me send messages to other people but i can call.every time i try to send a text it says free MSG: unable to send message - Message Blocking is active. I don't know what that means.

Try holding the home button and the power button down for a few seconds until it resets. That'll clean out any blockage in the service; Or fix any simple problem it might have.

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