This is a widely used term for the ability to use the internet to make and receive calls. This is acheived either as a SIP trunk onto an existing telephone system or using a Hosted Telephone system using Internet Protocol (IP).
Most modern telephone systems are IP ready/enabled to use SIP trunks, some even have the ability to use VoIP phones on the system for extension use.
Hosted VoIP uses a telephone system that is hosted in centres based throughout the UK. Hosted telephony is becoming more and more popular as better broadband (including FTTC) becomes available in more locations and areas.
Bandwidth is extremely important in VoIP as each telephone conversation uses about 85 – 100 Kbps in both directions.
So ordinary ADSL, whilst OK for low number of users, is not as good as FTTC.