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Cisco IP Telephony Network Design Guide

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Chapter 10 Migrating to an IP Telephony Network

Reference Models for Migration Configurations

There are three important requirements for IP telephony applications in model B

networks:

• When a party calls an IP phone and the call is forwarded to voice mail, the

caller should hear the IP user’s greeting for call answering. This can be a

problem because, if the PBX sees the call from the IP network as a trunk call,

it might not preserve the original called number on the call. In this case, the

caller would hear the general greeting (for example, “Welcome to Cisco”).

• When IP users press their message key, they should be prompted for their

password. That is, the voice messaging system should be passed the

information to associate the call with a user’s calling number to identify the

right mailbox.

• The MWI on the IP phone should be switched on and off to reflect the state

of the user’s voice mailbox.

In general, none of these three features can be achieved in a simple type B system,

where the link between the IP network and the PBX is PRI, and the configuration

sequence for a type A system is used. However, by using a more complex

configuration change on the PBX, the first two features can be achieved.

This model B implementation essentially requires configuring a phantom

telephone user on the PBX. For ease of maintenance, it is convenient to choose a

block of DNs that relate to the IP user’s DN. For example, for IP DNs 32XX,

create equivalent PBX phantom users of 52XX. The phantom phone is

permanently forwarded to voice messaging. On the IP network, the phone is

configured to forward to the phantom DN for voice messaging, with a speed-dial

key on the phone to dial the phantom DN. This key can be labeled for voice

messaging (except on the Cisco IP Phone 7960). Now, both call answering and

message retrieval calls go straight to the user’s voice mailbox.

There are drawbacks with this workaround. It requires extra administration and

user effort, and the IP user’s voice mailbox must have a different number from the

DN of the phone. Also, on some PBXs, it is necessary to configure real line cards

for the phantom phones. Perhaps it would be easier to administer if the user’s PBX

DN were retained on the PBX as the phantom DN during migration, and the user

could be assigned a new DN on the IP network. The original DID (DDI) could be

retained if the trunks are switched to the IP network and an incoming digit

translation is used. However, this would perpetuate a situation in which the user’s

DN, DID (DDI), and voice mailbox number do not match.

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Cisco WS-X4604-GWY - VoIP Gateway Especificaciones Pagina 215