Voice over IP

As internal networks began to grow, Voice over IP (VoIP) based PBXs began to emerge. Instead of using traditional analog lines to connect internal users, the VoIP handsets connected to the PBX over the IP protocol, just like a computer or any other device on the network. This allowed voice and data traffic to share a common infrastructure, which cuts down on wiring and management overheard.

Just like with traditional PBXs, VoIP requires some form of signaling to control the calls. An early form of signaling used for VoIP was H.323, and the Media Gateway Control Protocol (MGCP) has also gained widespread adoption.

The Session Initiation Protocol, or SIP, has also emerged as a standard that many IP PBXs use for signaling. Lync Server uses SIP for all of its internal signaling and for integrations with other PBX vendors because it provides a common framework for controlling calls. Vendors can also implement extensions on top of SIP to provide additional signaling capabilities. These extensions make SIP extremely flexible, but can lead to interoperability problems between different IP PBXs because each vendor develops its own extensions.