UCM 8.6.2a - migrated to USC-C (Issue occurred prior to migration)
Cisco 7921/7925 phones periodically fail to re-register and end with 'not in service' error. While troubleshooting we discovered that by changing the IP address of the device (via dhcp reservation) we could get the phone to register and would continue to work in most cases. We first thought it was a wireless problem but were able to reproduce the problem by attaching directly to the vlan where the wireless segment terminates and assigning the suspect IP address. During the issue, I can ping the device and see it attempt to download files via the TFTP server. It appears to be something with the specific IP address that was previously registered or the associated session.
I have a TAC case opened but wanted to get some feedback from others to see if anyone has experienced anything similar. Thanks in advance.
Try recreating the phone in CUCM. Delete the MAC or just create a false mac for the existing phone, copy the phone profile and add the correct MAC. Config it as you need to. See if that helps.
I have deleted the phone and rebuilt it, the same IP address is assigned based on the MAC and the problem continues. It's strange, I can change the reservation to a MAC of another phone and the issue follows the IP address. I have also downgraded the phone load to the previous version without success.