Business telephony
PABX
Acronym: PABX
A PABX is an organisation's private telephone exchange: it routes incoming calls to internal extensions and groups outgoing ones.
It explains why an internal extension is reached by a short number and why a prefix is sometimes needed to dial out. Many corporate fax machines sit behind a PABX, which adds one link between the device and the line — and one more place where a transmission can fail without the line being at fault.