It's simple: Voice becomes another data application running over the IP network. The PBX becomes a large server.
Technically, a VoIP call takes place in the following way:
A user picks up the handset, which signals an off-hook condition to the signaling application portion of the VoIP in the network. Then the session application portion of VoIP issues a dialtone and waits for the remote client to dial the number. When a number is dialed, the numbers are stored by the session application. After a sufficient number of digits are accumulated, the number is mapped to an IP host through the dial plan mapper. The IP host has a direct connection to either the destination telephone number or a PBX to complete the call.
Next, the H.323 ...