A small number of 7921G phones were programmed with an incorrect hardware revision of 5.6.
A 7921G phone received as a replacement via RMA fails to upgrade from CP7921G-MFG-F.2.LOADS.
Each time the phone downloads its configuration file from TFTP the phone will go through the process of downloading the new phone firmware. The new load will not be installed because the phone's hardware revision is not supported by the new firmware.
The phone will not be registered while it is downloading the new firmware and thus will not be able to make or receive calls. After the firmware upgrade fails the phone will proceed to register as expected.
Note: The MFG-F.2 firmware is functionally equivalent to the 1.4(3)SR1 firmware posted on cisco.com. The phone will operate normally on this manufacturing load.
For all of these models they will indicate a hardware revision of 5.6. You can view the hardware revision of the phone by using a web browser to view the device information page of the phone's web page. Reference the 7921G Admin Guide for more information on this page.
Because the nature of wireless phones can lead to them frequently losing network connectivity to the CUCM server it is possible to end up with a phone that is constantly trying to upgrade to a new firmware, only to fail each time. To mitigate this in the phone's device page in the CUCM Admin page configure an affected phone to use a phone load of CP7921G-MFG-F.2. This should prevent the phone from constantly trying to upgrade its firmware until it can be sent back to Cisco for repair.
What to do if you have an affected phone
If you or one of your customers has a phone with this invalid hardware revision it must be sent back to Cisco via the RMA process for repair. Open a TAC Service Request to have your phone(s) replaced.