Iphone reported lost or stolen - but it's not

Hello All, 

I bought an iphone(6S) in Jan 2016 by paying off full amount from a ATT Store in NJ and have a receipt for that. And it was part of my ATT family plan.

Recently that phone(along with the number) was moved out of ATT for few weeks (Moved to T Mobile and it was working fine).

Then after few weeks in TMobile we tried moving back to ATT - and it gave an error message that the phone was reported lost/stolen. We called customer care multiple times and tried to fix it - and they said it's all fine - but whenever we goto a store it gives the same error message.

We called ATT customer care multiple times and went to the store multiple times, but it didnt work. ATT Fraud team says they don't have record of this phone reported lost/stolen - it only happens when we try to add this IMEI on ATT system to add a new line. 

Can someone help me on this please?