Why cant i text certain numbers from my phone? (HTC EVO Shift 4g)?

I have the HTC EVO shift 4g from Sprint and for some reason I cannot text certain numbers in my phone. I recently updated my phone and Idk if that has something to do with it but I can text everyone else in my phone except for like 2 numbers. And everything else works jus fine. I need some help please!!!!! :-(