We are trying to keep our current landline phone number so we bought a burner phone and 10 min SIM card with AT&T but the instructions only talk about TMobile. So do we need a TMobile SIM card for the burner phone?
I didn't know that AT&T Prepaid still sold by-the-minute service, but sure, you can use it. The procedure is exactly the same otherwise. Be sure to wait a full week after the landline port to the AT&T service, then submit the port request to Google Voice.
The essential thing you need to do is to contact AT&T Prepaid after you establish service, and say "I need to port my AT&T number out to a land line carrier. What are the account number and PIN I need to submit to the landline carrier, so that AT&T will approve the port request?"