Asterisk is a PBX

Asterisk is a Private Branch Exchange (PBX). A Private Branch Exchange (PBX) can be thought of as a private phone switchboard connecting to one or more telephones on one side, and usually connecting to one or more telephone lines on the other. This is usually more cost effective than leasing a telephone line for each telephone needed in a business.

Extension-to-Extension Calls

First, as a PBX, Asterisk offers extension-to-extension calls. This means users can dial from one phone to another phone. While this seems obvious, elementary phone systems are available (often referred to as Key Systems) that support multiple phones and multiple lines, and allow each phone to use any line.

In operation, the handsets do not have individual extensions that can be dialed, and so there is no way to initiate a call from one handset to another. These systems can usually be identified by having a blinking light for all outgoing lines on every telephone. Unlike Key Systems, Asterisk allows for extension-to-extension calls, allowing directed internal communications.

In the previous diagram, each extension (meaning everything to the left of the PBX) can connect to any other extension by dialing it directly. This means if a modem were to send a fax to a local fax machine, it would be done by creating a direct connection between the devices through the PBX.

Line Trunking

Secondly, Asterisk offers line trunking. In its simplest form, line trunking simply shares access to multiple telephone lines. These telephone lines are usually used to connect to the global telephone network, known as the Public Switched Telephone Network (PSTN). However, they can also be used as private lines for other phone systems.

These connections can be a single analog trunk, multiple analog trunks, or high-capacity digital connections that allow multiple concurrent calls to be carried on a single connection.

Telco Features

Asterisk supports all of the standard features we would expect from any telephone company (or telco). Asterisk supports sending and receiving caller ID and even allows us to route calls based on the caller ID. Using caller ID with the PSTN requires us to subscribe to that feature with our PSTN connection provider.

As expected, Asterisk also supports other features such as call waiting, call return (*69), distinctive ring, transferring calls, call forwarding, and so on. These basic features and more are provided by Asterisk.

Advanced Call Distribution

Asterisk can receive a phone call, look at attributes of the call, and based on that make routing decisions. If enough information is not supplied by our PSTN connection provider, we can ask the caller to input the information using a touch-tone phone.

Once we make a decision on how to route a call, we can send it to a single extension, a group of extensions, a recording, a voicemail box, or even a group of telephone agents who can roam from phone to phone. We can use call queues to serve our customers more effectively while maintaining operational efficiency.

This flexibility gives us the ability to move from having just a phone system, to creating powerful solutions that are accessed through the telephone. Advanced Call Distribution (ACD) empowers us to serve our customers in the best way possible.

One major differentiating factor between Asterisk and other PBX systems that support ACD is that Asterisk does not require the purchase of a special license to enable any of these features. For example, the limit on how many calls can be queued at a time is determined only by the hardware we use.

Call Detail Records

Asterisk keeps complete Call Detail Records (CDR). We can store this information in a flat file or preferably a database for efficient look up and storage. Using this information, we can monitor the usage of the Asterisk system, looking for patterns or anomalies that may have an impact on business.

We can compare these records to the bill that the phone company sends out. They allow us to analyze call traffic, say to run a report to find the ten most commonly-dialed phone numbers. We can also determine the exchange that calls us most frequently so that we can target our marketing to the right area.

Moreover, we can look at the time duration of each call. We can count the number of calls a specific agent answers and compare it with the average. There are many uses of this feature.

Using this information, we can also identify abuses of our long-distance calling service. Employees all around the world misuse long-distance call facilities provided by employers. Asterisk gives us the tools to detect possible misuse. The importance of calling records should not be underestimated. This information is invaluable for a variety of business functions. As many countries operate a national do-not-call list, we can quickly determine if we have called anyone on the list to ensure that our verification and checking processes are adequate.

Call Recording

Asterisk gives us the ability to record calls that are placed through the PBX. We can use this to provide training material, as examples of calls that went badly or went well. This can also be used to provide call content to satisfy customers or partners, which could potentially be helpful in a legal situation. It's important to consider this feature when setting up your Asterisk service, as you may have substantial hardware and storage issues to address if your PBX is destined to handle and record a substantial number of calls.

Asterisk provides this feature and it is up to us to determine if it is legal, appropriate, and helpful to use in particular circumstances.

Call Parking

For users still used to the old Key Systems, call parking is a great feature that allows you to take a call, place it into a parked slot, and then allow another person in the office to pick up that line by accessing the slot. This process mimics the old Key System approach where you pick up a call, place the caller on hold, and then communicate the line number to another person in the office. Instead of a line number, call parking will give an employee a slot number, which if dialed will allow you to pick up that parked call. The slot number will be communicated to the user transferring the caller into call parking, which is accessed by dialing the call parking feature code.

For example, let's say you receive a call in the front office, but you need to check on something in the back. You don't want to transfer the call to the back office because if nobody is there then the caller might end up in voicemail before you reach the phone. Call parking allows you to place the caller into a parked slot. A slot number will be communicated to you. Now you can take your time to go to the back office, pick up a phone, and dial the slot number. Once it is dialed, you will be reconnected with the caller.

Call Barging

This is an excellent feature for managers who are training new employees or for those who want to conduct quality assurance. Call barging allows a user to listen to another conversation currently in progress on the Asterisk server. Through Whisper mode, a manager can even communicate to his employee without the remote user hearing the conversation. This allows the manager to coach the employee on a live call without the customer knowing it.