Consumer VoIP
There are various consumer orientated VoIP services which provide free or low cost voice calling, either through a telephone-like program on the computer, or through a special adaptor which connects to a home broadband service. These services include those offered by BT Communicator, Skype, FreeTalk (Dixons Group) and Vonage.
Consumer VoIP services are ideal for making low cost calls to the public telephony network and many of them offer consumers excellent deals on calls, including unlimited UK calls and low cost international calling. They will also offer the ability to call other users within the "service" for free. For example, one Skype customer can call another Skype customer at no cost.
The downside of such a services is that, whilst typically orientated around a software program on the users computer, they are less intuitive to use than a regular telephone and if the computer process is busy then sound quality can suffer. Additionally, it is important to ensure the software is used with a good quality headset, rather than a microphone and speakers, as otherwise the sound can become distorted with feedback and background noise.

Small and medium size business VoIP
Many VoIP services fall into the "Centrex" bracket. Centrex is a phone service where the central call control and switching, typically done by the box on the wall in your office, is performed by a central server at a remote Internet hosting centre. It is, simply, a "PBX in the sky". To use the Centrex service, customers will have telephones on their desk which connect out to the PBX server across broadband Internet. This provides a number of benefits, including:

• Ease of deployment - phones simply need to be plugged into an Internet connection
• Reduced capital expenditure - no need to purchase expensive phone switches
• Reduced operational expenditure - maintenance of the phone service is now typically done via the web
• Reduced "internal" phone costs - particularly for inter-site calls, e.g. between offices and home workers
• Increased flexibility and new ways of working - home working, distributed offices etc

Large business VoIP
Perhaps the oldest element of the VoIP market place is the interconnection of the various traditional office phone systems, aka the "PBX" across the different corporate sites.
Many corporate customers have phone systems from vendors such as Siemens, Panasonic or Avaya and frequently through the use of a converter or VoIP adaptor these systems can be linked so that calls which route between different company sites can be carried over the corporate VPN or Internet connection, and as such, are carried at no additional cost.
Similarly, many new office phone systems are being delivered as "IP enabled" systems. For example, they may have built in multi-site connectivity using an Internet connection, or they may have support for home-worker handsets such that the handset can be connected to the head office PBX, but across the Internet.
Whilst the VoIP-enabling of legacy phone systems is not the focus here, it is worth noting that there are a number of vendors offering solutions and that there may be an "easy win" to be had simply by implementing a cost saving through the use of new technology. It is recommended that you discuss this sort of implementation with your current equipment maintainer in the first instance.