From a purely technical sense, one must consider the way the PSTN handles telephone calls now. Each call is set up and delivered from a pair of dedicated copper wires from the caller to the called party. This creates a dedicated physical connection for a single call with extremely good quality. The technology is sufficient to carry one call at a time over the pair of dedicated copper wires.
VoIP packetizes the voice turning it into data packets. These packets are then sent to the network. The same copper wire is now capable of carrying significantly more than one conversation. The same device used to send voice is also capable of sending other packetized information simultaneously like data, video and instant messages. So at the very least VoIP is a much more efficient medium to transport voice, and at the very best, will produce a much richer experience for the user when one considers the enhanced features of video and instant messaging.
For an example of an IP PBX go to IPitomy