I have ran into this issue before. Phone works at my office just fine, but when it got to the customers there was no audio. I know this sounds weird but I had to open ports on the remote side of the customers router. I would assign a static internal ip address on the phone and then have to open ports on the remote router to that IP address. Try walking a user through that in another state. For what its worth my problem was with comcast at the remote end.