Configuring the SIP Release Cause Code for Failed E9-1-1 Calls

When a Teams / Skype for Business client makes an emergency call, the call is routed through the Microsoft Mediation Server to the ELIN device, which sends it to the PSTN. In some scenarios, the call may not be established due to the destination (for example, busy or not found) or the ELIN device (for example, lack of resources or an internal error). In such a scenario, the Mediation Server requires that the ELIN device "reject" the call with a SIP release cause code 503 "Service Unavailable" (instead of the designated release call). Such a release cause code enables the Mediation Server to issue a failover to another entity (for example, another ELIN device), instead of retrying the call or returning the release call to the user.

To support this requirement, you can configure the ELIN device to send a 503 "Service Unavailable" release cause code instead of SIP 4xx if an emergency call cannot be established.

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To enable SIP response 503 upon failed E9-1-1:

1.

Open the Advanced Parameters page (Setup menu > Signaling & Media tab > Gateway folder > Gateway Advanced Parameters).

2.

From the 'Emergency Special Release Cause' drop-down list [EmergencySpecialReleaseCause], select Enable.

3.

Click Apply.