It really depends on what you want but we have been using Vonage since April and it's unbelievable. It's VOIP but doesn't need a computer! You just plug their gadget into your broadband router (that is already connected to the internet) and plug a bog-standard phone or fax into that. Free calls to any UK landline and very cheap mobile calls. The best part is that I take the gadget everywhere I go (anywhere in the world) and simply plug it into other internet connections (visiting my parents, hotels...). After a minute or so, it connects to the Vonage system and then anyone phoning our number (you can choose a dial code for virtually any UK city) gets through to me. Voicemail can be accessed by phone, or through the vonage website or (and this is my favourite bit) EMAILED TO YOU AS AN MP3!!!
Their fax line facility doesn't seem up to scratch but that could be our bandwidth issue (phone calls need only the slowest broadband connection).
And no, I don't work for them, I'm just obsessed!