Version 6.4 287 March 2012
SIP User's Manual 18. GW and IP to IP
18.4.7 Alternative Routing for IP-to-Tel Calls
The device supports alternative IP-to-Tel call routing, as described in this section.
18.4.7.1 Alternative Routing to Trunk upon Q.931 Call Release Cause Code
You can configure the device to do alternative IP-to-Tel call routing based on the received
ISDN Q.931 cause code. If an IP-to-Tel call is rejected or disconnected on the Tel side as a
result of a specific ISDN Q.931 release cause code that is listed in the Reasons for
Alternative Routing table (AltRouteCauseIP2Tel), the device searches for an alternative IP-
to-Tel routing rule in the Inbound IP Routing table and sends it to the alternative Trunk
Group. For example, you can enable alternative IP-to-Tel routing for scenarios where the
initial Tel destination is busy and a Q.931 Cause Code No. 17 is issued (or for other call
releases that issue the default Cause Code No. 3).
You can also configure a default release cause code that the device issues itself upon the
following scenarios:
The device initiates a call release whose cause is unknown
No free channels (i.e., busy) in the Trunk Group
No appropriate routing rule located in the Inbound IP Routing table to the Trunk Group
Phone number is not found in the Inbound IP Routing table
By default, it is set to Cause Code No. 3 (No Route to Destination). This default cause
code can be changed using the DefaultReleaseCause parameter. To enable alternative
routing using this cause code, you need to define this cause code in the
AltRouteCauseIP2Tel table.
Notes:
• You can configure up to two alternative routing rules in the Inbound IP
Routing table.
• If a Trunk is disconnected or not synchronized, the device issues itself
the internal Cause Code No. 27. This cause code is mapped (by default)
to SIP 502.
• The default release cause is described in the Q.931 notation and is
translated to corresponding SIP 40x or 50x values (e.g., Cause Code No.
3 to SIP 404, and Cause Code No. 34 to SIP 503).
• For analog interfaces: For information on mapping PSTN release causes
to SIP responses, see “Mapping PSTN Release Cause to SIP Response”
on page 282.
• For mapping SIP-to-Q.931 and Q.931-to-SIP release causes, see
“Configuring Release Cause Mapping” on page 267.
• For a list of SIP responses-Q.931 release cause mapping, see “Release
Reason Mapping” on page 242.
• For configuring IP-to-Tel routing rules in the Inbound IP Routing table,
see ''Configuring Inbound IP Routing Table'' on page 279.
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