An VOIP Telephone system makes sense because the circuit-based century-old telephone network (called the Public Switched Telephone Network, or PSTN) is primitive and inefficient. When a call is made between two parties on a traditional system, the connection is maintained both ways for the entire duration of the call, creating a circuit. Since one party is listening whilst the other is talking, at least half of the connection is wasted.
Instead of circuit switches, a VOIP telephone system relys on packet switching -- the basis of all data networks -- which send and retrieve data across the connection when it's needed, rather than keeping it open all of the time. It's much more efficient than circuit-switch networks, allowing more calls to share the same line. But more importantly, data networks already understand the technology. Since IP telephone systems communicate in the same way that computers do, there's no need to have a separate network dedicated just to voice calls.
So, a VOIP telephone system can bring about some obvious and not-so-obvious benefits for businesses. The clearest advantage is the vastly reduced cost of making calls.
IP telephony systems do not need dedicated switching hardware such as a PBX (private branch exchange); this can all be done in software, and are much simpler to manage than traditional phones. Closely integrating the phone system with business applications -- such as email and customer relationship management -- presents new customer interaction, employee productivity and marketing opportunities, and also provides what where once considered "premium" services -- such as caller ID -- as standard features. Day to day maintenance costs are also reduced since there is only one network to manage, and support services can be performed remotely.
Sales of these systems are now an option from the traditional PBXs. Many companies are implementing IP telephony as the opportunity arises, such as for greenfield sites, whilst others are running some IP telephony services -- such as installing an IP PBX when their existing PBX needs replacing -- in tandem with their existing systems. IP telephony may not have an immediately obvious impact to the way we communicate at work and at home as other recent technology developments, but it could have the biggest impact since the PC replaced the typewriter