Version 6.6 325 Mediant 2000
User's Manual 24. Routing
24.5.3 PSTN Fallback
The PSTN Fallback feature enables the device to re-route a Tel-to-IP call to the legacy
PSTN using one of its trunks if the IP destination is unavailable. For example, if poor voice
quality is detected over the IP network, the device attempts to re-route the call to the
PSTN.
The steps for configuring alternative Tel-to-IP routing to the legacy PSTN are summarized
below.
To configure alternative Tel-to-IP routing to the legacy PSTN:
1. Configure an alternative routing rule in the Outbound IP Routing table with the same
call matching characteristics (e.g., phone number destination), but where the
destination is the IP address of the device itself.
2. Configure an IP-to-Tel routing rule in the Inbound IP Routing table to route calls
received from the device (i.e., its IP address) to a specific Trunk Group connected to
the PSTN. This configuration is necessary as the re-routed call is now considered an
IP-to-Tel call. For configuring IP-to-Tel routing rules, see 'Configuring the Inbound IP
Routing Table' on page 317.
24.6 Alternative Routing for IP-to-Tel Calls
The device supports alternative IP-to-Tel call routing, as described in this section.
24.6.1 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, 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 received (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 'Default Release Cause' parameter located in the
Advanced Parameters page (Configuration tab > VoIP menu > SIP Definitions >
Advanced Parameters). To enable alternative routing based on Q.931 cause code, you
need to define this cause code in the Reasons for Alternative Routing table.
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