I recently purchased a used Cisco 7970 phone and wanted to
convert it to use the SIP protocol. I did a factory reset and soon
discovered that one cannot upgrade from the factory state
to a recent firmware build without first loading an intermediate firmware.
The problem is that the firmware in the phone validates any new firmware upload before installing it, and it does it using built-in authentication keys. Somewhere along the line Cisco changed the authentication keys used to validate the firmware, and old firmware will refuse to validate new firmware. In between, however, there were a few firmware versions that included the keys for both old and new. SIP firmware versions of those dual-key firmwares, however, are not readily available.
The problem is that while both old and new SIP firmwares can be downloaded for free, the intermediate "bridge" builds and not available from Cisco for free and require a paid Cisco account and I didn't want to get one just for this experiment.
I found a workaround by going through a pair of free SCCP firmwares (i.e. non-SIP software). The following sequence of re-flashes can probably be optimized, but it was faster for me to just go ahead this way.
Upgrading my phone directly to firmware version 8.5 (cmterm 8-5-2SR1) or any newer version led to an "AUTH FAIL" error. Here's what does work:
This is also available free from Cisco. The actual zip package is cmterm-7970_7971-sccp.8-5-3.zip The important point is that this firmware includes the authentication keys for new builds! The corresponding SIP firmware cannot be download for free, incidentally.
Look for it at https://software.cisco.com/download/release.html?mdfid=278436620&softwareid=282074288&release=9.4(2)SR1
Note that if you need to do all this via a TFTP server. For each
phase unzip the corresponding firmware package, modify the XMLDefault.cnf.xml
to indicate the loads file to use, and then reboot the phone.