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Chapter 6
Setting up VoIP trunks for outgoing calls
113
Describing a fallback network
The following figure shows how a fallback network would be set up between two sites.
Figure 33 PSTN fallback diagram
IP network
Business Communications
Manager B
Business
Communications
Manager A
PSTN
Public or Private
PSTN line
Public or Private
PSTN line
In a network configured for PSTN fallback, there are two connections between a Business
Communications Manager and a remote system.
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•
One connection is a VoIP trunk connection through the IP network.
The fallback line is a PSTN line, which can be the public lines or a dedicated T1, BRI, PRI or
analog line (E&M), to the other system.
When a user dials the destination code, the system checks first to see if the connection between the
two systems can support an appropriate level of QoS. If it can, the call proceeds as normal over the
VoIP trunk. If the minimum acceptable level of QoS is not met, the call is routed over the second
route, through the PSTN line.
For PSTN fallback to work, you must ensure that the digits the user dials will be the same
regardless of whether the call is going over the VoIP trunk or the PSTN. In many cases, this
involves configuring the system to add and/or absorb digits. This process is explained during the
steps in “Configuring routes for fallback” on page 114 and “Adding the destination code for the
fallback route” on page 116.
For detailed information about inserting and absorbing digits, see the Programming Operations
Guide.
IP Telephony Configuration Guide