Mapping PSTN Release Cause to SIP Response for Analog Interfaces

When establishing an IP-to-Tel call, the following rules are applied:

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For all other FXS release types such as:

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no free channels in the Trunk Group,

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an appropriate call routing rule to a Trunk Group doesn’t exist, or

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the phone number isn’t found

the device sends a SIP response to the IP according to the 'Default Release Cause' parameter. The parameter defines Q.931 release causes. Its default value is 3, which is mapped to the SIP 404 response. By changing its value to 34, the SIP 503 response is sent. Other causes can be used as well.