Samsung G4 from Verizon won't always send texts using AT&T

[AC Question]

My wife recently was given an unlocked Verizon G4. I got an AT&T Sim for it and for the most part it works just dandy.

However it will receive but not send texts when she is at work. It will send them when she's home, I figured it had something to do with the WiFi, so I drove a couple blocks away and it still will send them. Just not when she's at work.

I've tried setting the APN using the settings at&t lists - to no success.

Thanks