Section 5.4. Messaging Applications

5.4. Messaging Applications

Telephony messaging applications are one-way in nature, distinguishing them from call-handling applications. That is, a message is sent from one party to another, and unlike a two-way phone call, the receiving party cannot respond in the same medium without initiating a return call. For example, a voice mail message is received. The receiving party must then make a call back to the sender in order to respond, if a response is necessary.

5.5.1. Overhead Paging

Overhead paging is when an endpoint broadcasts the voice of the caller over a large group of endpoints simultaneously . Most legacy overhead paging applications use public address speakers or ceiling speakers wired to a power amplifier. This PA is connected to an analog port on the PBXthe "paging channel"and the PBX sees the amplifier as an analog endpoint. Some PBX systems permit the use of TDM and analog telephones to be used as broadcast endpoints, too.

5.5.2. Barging

Barging is similar to overhead paging, but instead of broadcasting the caller's voice over a large group of endpoints, the caller can send his voice to a single extension. It's called barging because this feature tends to allow callers to interrupt calls in progress. Some PBX dialects call this feature announce .

5.5.3. Voice Mail

Voice mail is one of the most beloved telephony applications. Essentially, it records voice messages on behalf of people who aren't available to answer their calls. Practically everybody uses a form of voice mail, from the old-fashioned cassette tape answering machine all the way up to sophisticated multisubscriber voice mail services.

Some switches have voice mail built in, but most offload the voice mail functions to a dedicated server, usually a PC running a flavor of Unix, Windows, or even OS/2. Messages are stored as digitized sound files when the caller is prompted to speak. Later, the intended recipient plays back the voice mail messages from a phoneeither the phone attached to the same switch as the voice mail server or perhaps from a phone on the PSTN that has dialed access to the voice mail server.

5.5.3.1 Message notification (pager, email, etc.)

Some voice mail servers add the ability to notify each user when she has messages waiting. This may mean signaling the PBX to light an indication lamp on that user 's phone, or it may mean causing that user's phone to produce a stuttering, abnormal dial-tone when the receiver is off the hook. In either event, the idea is to notify the user that messages are awaiting an ear.

Some network-aware voice mail servers can send an email to a user when he has messages waiting, or even email the message itself, in the form of a WAV file, for example. Most voice mail servers support some kind of dialed notification as wellmeaning numeric or alphanumeric paging or an automatic agent that calls the user's cell phone to tell, via a recorded voice, that new messages are waiting to be heard . The user can then listen to the messages if he wants.

With some tweaking and a few additional software packages, a Linux-based Asterisk server can be set up to replicate all of these scenarios. In its default configuration, Asterisk can be used to automatically notify voice mail subscribers by email when they've received a callusing sendmail or qmail.