Looking for some insight on a Straight Talk issue.

Has anyone had any succes using an AT&T iPhone 4S on straight talk? My buddy claims he has the iphone unlocked, but is still having issues sending/receiving calls and texts, yet he is able to use data. This makes no sense to me (because I would expect it to be the other way around). I was hoping someone could point me in the right direction as to how to solve this issue for him.

The straight talk SIM is activated, and the APN is updated. He says the phone worked for roughly an hour before stopping, he is getting a "device has been blocked from network use" alert.

Anyone have any ideas what the issue is? I appreciate any help.

-K