IPhone 4 ios7- iMessages send when no service, only Wi-Fi?

I recently acquired an iPhone 4 from a friend who wasn't using it anymore. He was using AT&T, but I had it unlocked and I use it with T-Mobile. I'm not on a data plan, I use Wi-Fi where ever I have it. The problem is with iMessage- if I have bars for cell service and Wi-Fi, my iMessages will not send. When I don't have any cell service but I do have Wi-Fi, only then will they send. Any ideas why?

If defaults to cell service first is my guess.

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