I was looking into this issue for a customer where a Conference phone, Cisco 7936, was not registering to Call manager.
When I looked the MAC address of it in Call manager, it was in the status of ‘unregistered’ and there was an IP address assigned to it as well.
This suggests that it somehow reached to Call manager but then got unregistered OR this phone was registered to Call manager some time in back.
There were some other 7936 phones as well which were all registered fine.
I asked them to do a factory reset of the 7936 phone which they did but no joy. After that I asked them to disable DHCP and manually assign TFTP address but even that didn’t resolve the issue. Phone was stuck in a loop ‘TFTP server not found’.
Surprising bit was that both Call manager and Phone could ping each other that means there was no network issue. I asked them to hook another 7936 to the same port to see if that makes any difference. The other 7936 phone got registered fine. This proved the port was also fine.
I decided to jump onto the access switch where this phone is connected and see what messages it is sending. That proved a nightmare as we only had information regarding their main Inbound/outbound gateway. I had to track it down all the way from that gateway to distribution switches to that one particular access switch which was showing this MAC address connected to its port. I ran commands like debug tftp events and debug tftp packet but I didn’t see any response from the phone.
Frustrated I was, decided to google the issue. I found out that it’s a bug which has been hitting this phone.
Bug id# CSCsg95361
7936 phones fail to upgrade to a different load when upgrade is attempted immediately after re-registration to CCM. This situation happens when the phones unregister first due to callmanager getting restarted or upgraded.
The 7936 phones running either cmterm_7936.3-3-1-0.bin or cmterm_7936.3-3-2-0.bin loads can fail to upgrade to new load if the phones first unregister from CCM, followed by re-registration and firmware upgrade attempt.
Change the default load on the CCM to cmterm_7936.3-3-3-0.bin or newer and upgrade the 7936 phones without stopping CCM service first. If the phones experience this problem due to proactive workaround not performed, power cycling the phone recovers them in most cases.
In my case, nothing resolved the issue so I asked the customer to raise a RMA request with Cisco.
Also read this Field notice: