Large-scale firmware rollouts can saturate a network. Consider updating phones in batches rather than all at once.
This article breaks down what this specific file is, why the "SR" designation matters, and how to successfully apply the update to your Cisco 7975G IP phones. What is CMTERM-7975-SIP.9-4-2SR4.ZIP? cmterm7975sip942sr4zip upd
If the phone logs show an "Authentication Failed" error, ensure that your ITL/CTL files are synced correctly. Large-scale firmware rollouts can saturate a network