SIP
Softphone
STUN

SIP (Session Initiation Protocol)
rotocol for initiating an interactive user session that involves multimedia elements such as video, voice, chat, gaming, and virtual reality. SIP works in the Application layer of the OSI communications model. [+info]

Softphone (Software Telephone)
A PC or PC-type device that emulates the telephone with prompts and commands appearing on the PC's screen instead of the phone [+info]

STUN (Simple Traversal of UDP through NATs)
a network protocol which helps many types of software and hardware receive UDP data properly through home broadband routers that use network address translation (NAT).