We notice here that the caller can also have the benefit of the SIP presence
service. In this case, the presence service would not only notify the caller of the
availability of the called party anywhere on the PSTN/mobile or Internet, but
could also convey additional information about the willingness to accept calls
and other information, such as being in a quiet place (meeting) or already in a
call, and even who the other party in the conversation might be. Displaying
such information is also subject to the preferences of the called party.
All these communication capabilities argue against the use of a voice-centric
PBX, be it TDM- or IP-based.Residential User with ENUM Service
A residential user who wants to print only one residential phone number (for
example, the user may have separate phone numbers for the home and office)
on a business card has the following options:
■■ Request ENUM service from the local phone company. If the local tele-
phone company has ENUM service, the incoming call will be routed by
the intelligent network (IN) control system. The IN will use ENUM and
SIP to route the call, depending on the script for SIP called party
preferences.
■■ If the local phone company does not offer ENUM service, users can
request to have the telephone service moved to another service
provider that offers ENUM service. Local number portability (LNP) will
ensure the user keeps the same phone number. Incoming phone calls to
the user will now be diverted to the other service provider (the ENUM
service provider), using the Local routing number (LRN) to designate the
alternate terminating switch of the ENUM service provider. The ENUM
service provider will route the call as described previously.Notice that if the called PSTN device happens to be outside the local switch
calling area (the called user is traveling and wishes to get incoming phone or
fax calls directed to some remote place), the ENUM service provider can divert
the call from the PSTN network and route the call over the Internet, thus
reducing the cost of the call. This operation, known as PSTN call diversion will,
therefore, also benefit from ENUM.Miscellaneous: ENUM Lookup of the Display Name
The profitable use of Internet technology in the transition from the PSTN to the
Internet is illustrated by many innovative applications, such as using ENUM
to map a phone number to a display name that can be rendered by the display
on the SIP UA for the benefit of the called party [20]. This application avoids
the use of the existing PSTN databases and their associated higher cost for
new, IP-based service providers.76 Chapter 4