ATT iPhone can only text some numbers

Okay so my dad had a Tilt with ATT that work gave him. He was never able to send or receive texts on this phone. Now he has upgraded to an iPhone 4G through work. IT says he should be able to text now.

He can text his coworkers as well as my mother, who is on VZW. He can also text my brother, who is also on VZW, on the same plan. I am on the same plan too and he can't text me and he never gets my texts to him. I'm really confused. How can we remedy this?

Sent from my VZW Thunderbolt.