SIP Reason Header for Release Cause

The device supports the SIP Reason header according to RFC 3326. The Reason header describes the disconnection cause of a call:

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Sending Reason header: If a call is disconnected from the Tel side (ISDN), the Reason header contains the value of the received Q.850 cause in the appropriate message (BYE/CANCEL/final failure response) and sent to the IP side. If the call is disconnected because of a SIP reason, the Reason header is set to the appropriate SIP response.

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Receiving Reason header: If a call is disconnected from the IP side and the SIP message includes the Reason header, it is sent to the Tel side according to the following logic:

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If the Reason header includes a Q.850 cause, it is sent as is.

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If the Reason header includes a SIP response:

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If the message is a final response, the response status code is translated to Q.850 format and passed to ISDN.

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If the message isn’t a final response, it is translated to a Q.850 cause.

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When the Reason header is received twice (i.e., SIP Reason and Q.850), the Q.850 takes precedence over the SIP reason and is sent to the Tel side.