You will require some type of VoIP device. This may come in the form of:
1. IP Telephone. This is like a normal phone except that it communicates using an IP connection. These phones plug straight into your router or LAN hub. You can check out VoIPtalk's range of IP Telephones here
2. Softphone. This is an application that runs on your PC and uses your PC's soundcard for audio transmission.
3. VoIP Gateway. This device converts analogue traffic from your analogue phones into VoIP traffic. They are usually referred to as FXS (Foreign Exchange Station) devices. You can check out VoIPtalk's range of VoIP Gateways here
4. VoIP Adaptor. This device is similar to a VoIP Gateway device except that is designed to interface a single analogue phone. You can check out VoIPtalk's range of VoIP Adaptors here
In addition to a VoIP device you will require an Internet connection (preferably broadband) and an IP router.