What are PABX and PBX?

A private branch exchange (‘PBX’, ‘Private Business eXchange’ or ‘PABX’ for ‘Private Automatic Branch eXchange’) is a telephone system owned by a business, as opposed to one owned by a common carrier or by a telephone company.
A PABX connects all the business’s instruments separately to the public telephone network PSTN or ISDN. A PBX switchboard assists in that every internal call made is routed through the PABX (central switch).
The PBX switchboard is installed at a business’s premises and connects incoming calls between the extension telephones installed .The outside lines (trunk lines) are available for making and receiving calls external to the site (PSTN). Companies with more than one site can connect their PBXs together with trunk lines. PBX-like services can also be provided by equipment located off site at a central provider, delivering services over the public telephone network. This is known as a Private branch exchange Hosted PBX.