Simplicity Is the Key
One of the things to keep in mind when deploying a dial plan is simplicity.
You want the setup to be as simple as possible to minimize the
number of entries input into the Cisco CallManager (discussed later) and
the router. By configuring the system in this manner, you are able to add
a higher level of efficiency to the system, which can offer you better performance
and less support headaches.
To this end, there are some things that are just common sense, such
as only configuring dial plans for locations on the network (On-Net), and
calls that are destined to go externally (Off-Net) to use the PSTN gateway
access code (usually a 9, but can be defined). You will also want to
create contingency plans for internal calls to complete through the PSTN
if the network is congested. One thing that customers will always
demand is dial tone.