www.vonage.co.uk
These people provide incoming calls and call divert services- you can also have a variety of "normal" numbers, eg 0207 123 1234. You can also have a US number so if people in the US call you, they will only be charged at national rates- can be good for business users. With Vonage you can have an adaptor so you can use a normal phone with VOIP.
The main problem with only using VOIP is that if your connection dies, you lose your phone line too- could be problematic for calling the emergency services, or your ISP to report a broken connection.