I can receive but not send texts

Hey folks,

I'll be calling ATT again tomorrow, but I figured I'd give the community a shot at this problem.

As the title belies, I can received text messages but not send them. It started happening relatively recently but I can't remember when. I've tried restoring my phone from a backup as well as a fresh start. I've tried clearing network settings and reseating the sim card. I called ATT and they took off the text plan from my account and put it back on and that was supposed to fix the problem. It didn't. The problem is not isolated to a single cell tower as it happened in a few different places all over the state.

Any thoughts or similar situations? As to what could have brought this on, I think I've narrowed it down to either:

1. when I switched out phones (in warranty) at the apple store.
2. I had an employee discount applied to my account.

I'm pretty sure it's one of these two possibilities. I'm leaning much more heavily towards number 2 as I've reset the phone and all other network related functions work fine (ie calls, web, incoming texts).