This firmware, , is the final maintenance release for the 7975G series. While the phone is considered legacy (obsolete), this specific package is widely sought after by home-lab enthusiasts and IT administrators to "develop" or unlock the following features:
Deploying firmware cmterm-7975-sip.9-4-2SR4.zip is not a straightforward drag-and-drop process. It is intended for enterprise environments using CUCM or Cisco Unified Communications Manager Express (CME). cmterm7975sip942sr4zip 2021
Are you registering this phone to or a third-party SIP server like FreePBX? What current firmware version is installed on the phone? This firmware, , is the final maintenance release