
Chapter 44 VoIP trunk gateways 407
BCM 4.0 Networking Configuration Guide
Fallback to PSTN from VoIP trunks
Fallback is a feature that allows a call to progress when a VoIP trunk is unavailable or is not
providing adequate quality of service (QoS).
Refer to the information under “Describing a fallback network” on page 407 for details about
setting up fallback for VoIP trunks.
By enabling PSTN fallback on the Local Gateway IP Interface panels for SIP and H.323 trunks,
you allow the system to check the availability of a VoIP trunk, then switch the call to a PSTN line.
For the PSTN fallback to work on a suitable bandwidth, QoS monitor must be enabled and a
transmit threshold must be set. For QoS and transmit threshold settings refer to Table 96.
You use scheduling and destination codes to allow the call to switch from SIP or H.323 to a PSTN
line without requiring intervention by the user.
Use the dialing plan worksheet in the Programming Records to plan your dialing requirements so
you can pinpoint any dialing issues before you start programming. If you are programming an
existing system, you can look at what numbers the users are familiar with dialing, and you can
attempt to accommodate this familiarity into your destination codes plan.
On any IP gateway for which you want to allow fallback based on network quality, you need to
ensure that QoS monitor is enabled.
Describing a fallback network
Figure 131 shows how a fallback network would be set up between two sites.
Warning: QoS monitor must be turned on at both endpoints. To enable the QoS Monitor
select Configuration > Telephony Resources > IP Trunks > Routing Table.
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