We have a network of multiple phones in the office, all Cisco IP 7960 Series phones. One person in the office has an issue where the person on the other end of the call can not hear them speak. We switched out the phones with another at an un-used desk. This worked fine for a while. The problem has recurred only at her desk for quite some time.
My personal thought is that the issue is with the connection at the computer and the desk and not the phone but I am not sure. I didn't set any of the phones up but I am an informal IT person for the company with no technical training so be gentle with your help! :P