Switching to VoIP

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Dial-Plan and PBX Design

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private trunks. Usually this terminology is applied the same way in a VoIP network.

Many of the same considerations have to be made when creating a dial-plan:

• How many phones will there be in the network? Does the extension number

allow for enough digits for all of them?

• Will phone users dial a special digit (8 or 9) at the beginning of PSTN-bound

calls, or will the phone system just figure out how to route their calls?

• Will calling between private switches be possible?

• Will extension numbers be based on a mitigating or symbolic convention—such

as the organizational location of the phone—i.e., which office location it’s at, or

a desire to match a particular user’s DID phone number? (See the section “DID”

earlier in this chapter.)

• Will each user’s voice mail box number match her extension number?

• Will calls routed from a particular user to the PSTN always travel the same

trunk, or will the trunks be selected randomly when calls are placed?

Some of these mundane questions can have a big social and economic impact. Using

a long extension number (five or six digits) could be unpopular with users but neces-

sary because of the size or growth potential of the voice network. Routing outbound

calls to the PSTN on a user-per-line basis, in combination with ...