If you register with
Sipgate you can for free (most others charge) get a UK number eg 01234 123456 (you choose the area code, too, so it looks like "home"). When you are logged off it goes to voicemail, which can send you a free email with the message converted to a .wav file, and when you are logged on, it'll ring! Most free numbers from VoIP cos are 056x, so are not counted in a fixed/mobile free/cheap calls package.