If you're with BT, you need to tell them you want a VOIP (Voice Over Internet Protocol) phone. They are dead easy to set up, but you will need a router that can handle a VOIP phone (some have sockets built-in), or you can buy one of BT's recommended Cisco VOIP phones that simply plugs into your normal phone socket and can then be set up to receive / make internet calls.
Just a small word of warning - the old BT 2700HGV business hub has two VOIP sockets on the back, but it doesn't work well with modern broadband - I believe BT still offer them to customers but I would strongly recommend you don't get one. I used them for a long time, but then we started getting intermittent lagging on the connection. It was due to the fact the country's broadband system was upgraded, and it left this router behind in a technological sense. BT denied there was an issue but there is stuff all over the internet about how the 2700 doesn't work well anymore, so don't get talked into having one!
Their newer Hub 5 works very well but it doesn't have VOIP sockets. We have one of these now and I had to buy the VOIP phone to go with it. BT's Cisco VOIP phone is pretty good, but it has two drawbacks - firstly, it doesn't have an answer machine, and second, the handset is on a chord, so you can't wander round whilst talking to someone unless you put them on speakerphone (which does work well to be fair).
If you have a router with VOIP sockets, you don't need a special phone - you can use any normal handset, including one with an answer machine built-in. It is slightly annoying that BT have actually gone backwards IMO with their VOIP options by making a new router without VOIP sockets. Presumably this was always a cunning ploy to make us buy the handset.
The good news is, you don't have to buy your VOIP phone from BT. They sell Cisco VOIP phones and you can get refurbs for as little as £27.50 (the telecomshop.com) elsewhere on the web. There are also other makes that are probably just as good.
The phone number will start with 05, and that tends to confuse people. For an extra fiver a month, BT can allocate a local area STD number for you and I would recommend you consider that as it gives customers a lot more confidence when calling. One plus point here is that, if you move, you can take your number with you, regardless of where you go in the UK. We moved last year from one STD area to another but I still have the old STD number because with VOIP, normal rules of numbers don't apply!
