Iphone problem! I can't get text messages from some people, but can from others?

I have a iphone 4, running on the latest version; ios6.0.1? I believe if I'm not mistaken.
I use it on t-mobile, i believe it is factory unlocked. I do use imessage, i attempted to turn it off and see if the person could text me again; still haven't received.
I do receive text message from some people. I restarted my phone a couple of times.
has anyone had this problem? If so what should i problem check to try to fix

Easy fix, go get an HTC One X+

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