Sometimes these phones will be shipped with a firmware load named similar to CP7925G-MFG-F.2 (see the table posted by mgilles below), which is particular in that
It can´t be downgraded to anything lower than 1.4(3)SR1
So, what to do?? Just make sure to upload a firmware equal or higher than 1.4(3)SR1, like the one shown in the Recovery Procedure picture.
Only in case of firmware corruption should the recovery method be used, following the steps below.
Some clarifications about the normal firmware upgrade (plugging the phone to the WLAN and let it talk to the TFTP server)
If by some reason you don´t see the Firmware Update option to the left of the phone´s web page, its because the web server is in read only mode. To enable it, find the device in Callmanager´s web interface, and under the Product Specific Configuration Layout, modify the Web Access field to "Full":
Recovery Procedure (when there is a corrupt image only, like booting issues)
Download the USB driver from this link (its the main download section for the 7921 model in case it breaks), the option you want to click on is the "IP Phone Tools and Utilities", and then you choose to download "USB-Install-792x.1-0-3.exe". This executables works for both models.
Enter into Recovery Mode and upload the special TAR bundled firmware as instructed starting in page 122 of the following PDF file
Some clarifications about the Recovery Procedure
The driver is only supported in Windows XP. Some users report that in Windows 7 the device driver is not recognized. Some others run it just fine.
The document speaks of an "application button", thats the top button on the left side when facing the phone's screen.
Works for 7921 and 7925.
Ensure that the USB driver has been installed (step 1 above).
At some point you will need to connect to the IP address of the phone through HTTP, here is how:
The USB driver automatically adds another network adapter and statically configures it for network 192.168.1.0. This is how you will be able to connect to the IP Phone through the USB using TCP/IP.
The IP Phone will communicate with your computer, by using IP 192.168.1.100.
Navigate on your web browser to http://192.168.1.100, and that will open a webpage hosted by the IP Phone where the only option is to browse for a TAR file, this is only when in Recovery Mode, and its the way we can push a different load to it
When choosing the firmware, remember to download the TAR file require during the Recovery Mode: