Pre-empting Existing Calls for E9-1-1 Calls

If the ELIN device receives an E9-1-1 call from the IP network and there are unavailable channels (for example, all busy), the device immediately terminates one of the non-E9-1-1 calls (arbitrary) and accepts the E9-1-1 call on the freed-up channel:

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SBC application: Preemption is done only on a call belonging to the same source IP Group from which the E9-1-1 call is received, or the same destination IP Group (i.e., PSAP Server).

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Gateway application: Preemption is done only on a channel belonging to the same Trunk Group for which the E9-1-1 call was initially destined. For example, if an E9-1-1 call is destined for Trunk Group #2 and all the channels belonging to this group are busy, the device terminates one of the calls in this group to free a channel for accepting the E9-1-1 call.

This feature is initiated only if the received SIP INVITE message contains a Priority header set to "emergency", as shown below:

Priority: emergency