PBX Enterprise Voice Network
The caller in the enterprise network on the left of Figure 4.5 tries to reach a
client with the phone number 214-123-4589. Suppose that there is some
urgency and using voicemail or e-mail is not desirable. There are several
scenarios:
■■ The enterprise PBX of the caller has not been upgraded for ENUM ser-
vice. The called party can only be reached using a phone number. In
this case, the call will be routed over the PSTN.
■■ The PBX has been upgraded for access to ENUM service by using a ser-
vice provider with an outgoing SIP proxy that is ENUM-enabled. In this
case, when dialing the phone number:
■■ The called party can be reached at any PSTN/mobile phone, with-
out the calling party having to notice the difference. If the called
party has also the benefit of SIP service, the caller may be notified
where the call has been redirected. This option is, however, under
control of the called party and can be made dependent on who the
caller is, location, time of day, and so on.
■■ The called party is not available. A voice announcement can inform
the caller of the alternative contact options. If paging is an option,
the called party can be paged and a IM text message sent by the
caller. If both the caller and called party have IM, they can start com-
municating, even if the called party is on another call.Enterprise System with IP Communications
The PBX has been replaced with a SIP-based IP multimedia communication
system (Figure 4.6). The callee can be reached anywhere on the PSTN or
mobile phone network or on the Internet, without the need for other systems,
such as a separate paging system. The ENUM system will provide a URI for
routing the SIP call to any destination.
The SIP phone user will be connected to the called party for a voice call, if
the called party is reachable on any PSTN/mobile phone or by an IP phone.
If the called party is in a meeting and has a laptop computer connected to
the Internet, the call will be redirected to the instant messaging client, and both
parties can use text chat in a nonintrusive manner. The caller in the enterprise
network can also push web pages or transmit other documents to the client
during the conversation.74 Chapter 4