Compared to later 9.4(3) builds, SR4 has less forgiving retransmission timers. If your WAN has latency above 150ms, you may see sporadic "Service Unavailable" messages.
Before discussing features or installation, let’s perform a forensic analysis of the filename itself. Cisco follows a strict naming convention for its phone firmware files, and cmterm-7975-sip.9-4-2sr4 is a textbook example. cmterm-7975-sip.9-4-2sr4
| Component | Value | Meaning |
|-----------|-------|---------|
| Prefix | cmterm | Indicates this is a Cisco Media Termination firmware file, typically used with CUCM or CME (Cisco Unified Communications Manager Express). |
| Model | 7975 | Specifies the target hardware: Cisco Unified IP Phone 7975G, a high-end color touchscreen phone from the 7970 series. |
| Protocol | sip | Denotes Session Initiation Protocol—the open-standard signaling protocol, as opposed to SCCP (Skinny Client Control Protocol). |
| Version | 9.4.2 | The base firmware version. Major release 9, minor release 4, maintenance build 2. |
| Sub-release | sr4 | Service Release 4 – an incremental update that includes bug fixes and security patches without changing the major version number. | Compared to later 9
Thus, cmterm-7975-sip.9-4-2sr4 is the fourth service release of SIP firmware version 9.4.2 for the Cisco 7975G phone. Deploying cmterm-7975-sip
Deploying cmterm-7975-sip.9-4-2sr4 requires a Cisco Unified Communications Manager (CUCM) version 8.x, 9.x, or 10.x. It is not compatible with CUCM 12.x or later without downgrading device packs.