You got me on this one. The phone has to be active on the other cell service in order to port it back to verizon wireless.

If you call AT&T they have to release the line number to the carrier you desire. But all that is normally required is the name of the carrier your porting from, your account number at AT&T, and your account password from AT&T.

I suspect you got a rep who doesn't know what they are doing.

Call them again and be insistent that should work.

Good Luck