Leo says to first make sure all the filters are installed. If that isn't the issue, then it may be the wiring in the house. The lines could have noise. The phone companies hate DSL Extreme because they are forced to work with them. So they'll likely blame DSL Extreme, even if it's actually the phone company's fault. If it's anything outside the house, it's AT&Ts problem and by law, they have to fix it.
There's an outside box that he could connect to. He can try getting a splitter from the box outside, then plug one end into the DSL modem, the other side into a DSL filter and then into a phone. The quality of the voice calls should be 100%. If it's not good there, then the problem is AT&T's. If it sounds good there, then it's the wiring inside the house.