You can check for the programming software.

But that’s ok for now.

You need to deprogram the ATT 1st in the ATT-1 Colum. Then reassign any EXT you want in the Answering Position Colum for ATT-1. Just make sure for trunks are assign to the proper ATT in private trunk features or DID Routing. Then put a NG button on that answering position set. You will probably have to make a new MAP for that EXT.