Nortel Networks Remote Gateway 50 - Manual (Page 169)

Dialing Plan configuration Page 169 of 258
Main Office Configuration for Survivable Remote Gateway 50 Configuration Guide
However, this configuration represents only one way that the dialing plan
could be configured. PSTN calls can be routed according to the point of origin
(main office or branch office) and/or the desired destination, and can select
trunks at the main office, branch office, or other branch offices as required.
Therefore, the user can route calls to gateways that minimize long-distance
costs, minimize bandwidth usage, or meet other criteria.
Nortel recommends that customers use Coordinated Dialing Plan (CDP)
between the main office and its branch offices since it enables all users, at the
main office or the branch office, to call each other using just an extension
number. CDP enables consistent dialing between the main office and SRG
IP Phones and devices.
For more information, refer to Dialing Plans: Description (553-3001-183).
Off-net dialing plan
When dialing to the PSTN, the Call Server determines that the call destination
is off-net by analyzing the digits that must be pre-configured at major Call
Servers in the network.
If routed over a Virtual Trunk, a request is sent to the NRS to determine the
location of public E.164 numbers. The NRS is configured with a list of
potential alternate routes that can be used to reach a certain dialed number.
Each route is configured with a unique “route cost” to determine the least-cost
route.
The NRS replies with the address information for E.164 numbers. It also
provides a list of alternative NRS (H.323 Gateway) endpoints, sorted by cost.
If a terminating endpoint resource is busy when a call attempt is made, the
originating endpoint tries the next alternative. If no alternative is available
over the IP network, the originating endpoint steps to the next entry on its
route list, which could be a TIE or PSTN alternate route.