My iphone 4 won't send texts?

I got the new IOS 6 update, and now that i've done that, i can't send text messages. I can send iMessage, but not texts. I can receive texts/imessages. But i can't send.
(Sprint iphone 4)
Anyone have a idea on how to fix it?

Have pu checked the settings for your messages? Maybe something switched around.

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