Voice over Internet Protocol
Voice over Internet Protocol (VoIP) is a very wide term for a range of transmission technologies for delivery of voice communications over IP networks.
Other terms frequently encountered and synonymous with VoIP are IP telephony, Internet telephony, voice over broadband (VoBB), broadband telephony, and broadband phone.
The plain old telephone network
VoIP systems usually interface with the traditional public switched telephone network (PSTN) to allow for transparent phone communications worldwide.
SIP and IAX2 are the most commonly used standards.
The SipBroker VOIP catalog has become somewhat of a standard for VOIP-provider to VOIP-provider calls.
Asterisk is the most popular PBX software for GNU systems.