External Calls
Configuring Cisco CallManager to complete external calls requires implementing
a route pattern.A route pattern is used to direct calls off network to a PSTN
gateway. Route patterns can also be used if there are Cisco CallManagers located
on a WAN-connected network.
Cisco CallManager dial plans are usually deployed in a tier system.This
system lets different routes handle dialed numbers.You can also manipulate dialed
strings, based on network requirements.This manipulation can either add or subtract
digits from the number dialed by the user so as to accommodate network
and gateway needs. Cisco CallManager can also create Trunk groups that will
handle redundancy and create better paths for least-cost routing. For example,
when using trunk groups, the system has the ability to choose an alternate route
to complete (or in some cases admit) calls if the trunks do not have sufficient
bandwidth to handle the call.This can be denoted (when creating the dial plan)
as a continuation of the rule that moves calls to the PSTN if WAN connections
are saturated.
In Figure 9.3, a call is placed from a telephony device (A). It is then matched
against the route pattern (1), where digit manipulation takes place. From here, the
call is forwarded to the route list (2).The route list adds the preference of connecting
the call over the WAN link. If the call is unable to be completed through
the WAN (because of insufficient resources or some other reason), then the call
will be forwarded to the PSTN. If the PSTN cannot complete the connection,
then the user will receive a busy signal (unless there is a third route configured).
From either the WAN or the PSTN, the call is forwarded to the destination party
(B). Again, this entire process should be transparent to the end user.