Cisco Emergency Responder (CER) and srst are different things.
First lets start with SRST.
In the event of a server outage phone register to the local gateway in SRST. SRST should be implemented anywhere voip phones would be considered a life safety issue. The local gateway should have PSTN access that when 911 is called the calls are routed to the proper PSAP and the PSAP sees ANI (Calling number) and the location of the call. This should be tested from time to time to ensure it is working properly, and someone didn't mess up the config in CUCM or local gateway.
This requires a local gateway per building.
I would often make that case that SRST should almost always be configured.
Cisco Emergency Responder or the scaled back version in newer CUCM version is different all together and provides more robust way to ID where the 911 call was placed from. Often this can be tied to switch ports or other methods.
As far as the legal requirements in the US it can vary based on the state.
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