Private Branch Exchange or PBX refers to a telephony switch that serves a specific office or business, as compared to an exchange that a carrier or phone company provides for different entities or the public.

PBX is also equivalent to PABX or Private Automatic Branch eXchange, and EPABX or Electronic Private Automatic Branch eXchange.

PBXs create connections between the organization’s internal telephones and link to the Public Switched Telephone Network PSTN) through trunk lines The term “extension” refers to an endpoint on the branch of the PBX; comprising telephones, modems, fax machines, etc.

Every telephone connects to the system in the PBX. When a user picks the receiver up and dials the access code, the system connects the user to an outside line.

PBXs vary as “key systems” as its users select their outgoing lines manually, while PBXs automatically connect to the outgoing number. Hybrid systems are a combination of the PBX and key systems.

Organizations use PBXs to lessen the telephone lines leased from a telephone company. Many medium- and large-scale companies use a PBX for the number to dial is usually composed of just a few digits.

The Centrex is an innovation on the PBX scheme, with switching occurring in a separate telephone office instead of within company premises.

Using the PBX system, the organization only requires a number of lines from the telephone service provider as the most number of people connecting to outside numbers at any given time.

Common PBX Systems:

  • Avaya’s Definity series
  • Northern Telecom’s Meridian series

Open Source PBX Systems

Many comprehensive PBX systems are open-source software. Significant networks supply product support and these systems are available at no cost to the user.