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Introduction to PBX

PBX
Martin Collier

By Martin Collier Consulant, Updated 17 May 2017

Rather than individual phone lines, most enterprises have a system that allows them to make and receive calls via a central system supporting a number of extensions. This is known as a Private Branch Exchange (PBX).

A PBX allows external lines to be shared amongst a number of people on the principle that not all phones will be in use at the same time. This saves on the number of lines you need with consequently lower costs. It also allows employees to communicate internally between extensions, and makes for efficient routing of incoming call traffic.

The term 'Private' in PBX comes from the fact that the equipment is owned internally by the business rather than by the company supplying the telephone service.

This introduction to PBX explains its features and alternatives.

Elements of a PBX

A modern digital PBX is made up of a number of elements. These include the PBX itself, a unit that switches calls within and outside the system. This is essentially a computer and may have a console - screen and keyboard - for management tasks. In many modern systems though, configuration and management can be carried out remotely using a PC. Management options include being able to assign levels of function to different extensions so that certain phones may be barred from making international calls or from being used outside office hours for example.

There will also be one or more operator consoles, often referred to as a switchboard, which receive incoming calls and allow them to be diverted around the system. Most will also have a remote service mode, for night time for example, which will divert general calls to an answerphone. Many systems have a direct dial in (DDI) function which allows extensions to have their own external phone number so that people can call them directly without going through the switchboard.

The other key part of a PBX system is the lines. There will be a telephone trunk line of connections entering the premises - this is often in the form of an ISDN line. Internally there will be a network of lines to individual desktop phones known as extensions. This uses the same type of cabling as Ethernet computer networks and can share the same infrastructure and outlets which makes for easy configuration and flexibility internally. Systems usually permit grouping of phones allowing a user to key in a code to pick up a call that is ringing on a nearby extension for example, or the use of hunting groups where a call will switch to another nearby handset after it has gone unanswered for a certain number of rings.

Alternatives to PBX

There are alternatives to a PBX in the form of centrex systems that outsource the PBX functions to your telecoms supplier. This means no equipment is required internally, but you still get all of the functions including dialling in and out, and transferring calls. It is managed by the supplier so there are no internal support requirements.

Another recent alternative is VoIP calling using the internet rather than conventional call lines. This can replace a PBX completely but can also be integrated into a PBX system allowing, for example, international calls to be made at a lower rate, or outgoing calls to be made on VoIP while incoming calls are handled in the conventional way.

VoIP system can also make use of what is called a soft PBX. This is a system that runs on a computer and provides the functions of a PBX without the need for dedicated hardware. IP phones can be used so staff can still have a handset on their desk. An added advantage of softphone systems is that they can be accessed remotely, allowing the business phone system to be used, for example, by staff working from home.

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