If in the middle of a call on the 7960, and a 7920 is powered on, everything is fine. The instant the call on the 7960 is disconnected, the phone un-registers and cannot re-register.
This issue has been seen with Non-Cisco access points (APs). Technically, Cisco does not support the 7920s with non-Cisco APs because of issues like this.
Usage of 7920 IP phones with Linksys Access Points is not supported. As a workaround, you can disable Cisco Discovery Protocol (CDP) on the 7920. In order to disable CDP on the 7920, complete these steps: