Few fields are as rife with jargon and abbreviations. Here's a guide to some of the terms you will encounter:

Centrex: a PBX (see below) hosted at the local telephone exchange

DID: Direct inward dialing; lets inbound calls go directly to extensions without an operator

PBX: Private branch exchange, a telephone switchboard used exclusively by a private organization

POTS: Plain old telephone service, i.e., a standard analog line

PSTN: Public switched telephone network, the standard network

QoS: Quality of service, an Internet protocol that gives voice traffic priority over data traffic

TDM: Time division multiplexing, a label for traditional, circuit-switched telephone equipment

VoIP: Voice over Internet protocol, a standard for transmitting voice communication via the Internet

vPBX: Virtual PBX, a scaled-down service for inbound calls