Most likely this is caused by a phone running on a firmware older than 8.5(2) being upgraded to something newer. This is a known issue, documented in the release notes for SCCP firmware release 8.5.(2)SR1:
For all SCCP firmware upgrades from firmware release versions earlier
than 8.3(3) to version 8.5(2)SR1 or greater, you must first upgrade
your firmware to version 8.5(2). Once you have upgraded to version
8.5(2), you can upgrade your IP Phone to version 8.5(2)SR1 or later.
So, assuming you're using CUCM, you can approach this several ways.
I recommend uploading the 8.5(2) firmware files to your TFTP server. Once you have the 8.5(2) firmware on your TFTP server, you can do the following (Cisco has a more in depth "How-To" located on their Support Forums):
- Open device in CUCM, find the field "Phone Load Name" under Device Information.
- Manually assign the phone to use 8.5(2) firmware by entering "SCCPXX.8-5-2S" where XX is the model of the phone. (11 for 7911, 42 for 7942, etc. but note that 7962 and 7965 models still use the "42" or "45" firmware respectively.)
- Apply configuration and allow phone to upgrade to 8.5(2).
- Remove manual Phone Load entry, apply configuration again and phone will upgrade to current release specified globally in CUCM.
This will allow you to resolve the issue on a phone-by-phone basis.