We run multiple phones using SIP, look up a few of the suppliers SIPgate.co.uk are one of our providers, basically Google for SIP and VOIP. Both SIP and VOIP route your calls via the internet, like Skype and similar.
I've tried a few systems including a Fritz!Box ADSL router that had built in SIP clients support for DECT cordless phones, Draytek do a similar model. The latest one I'm using, because it's so simple is the Siemens Gigaset N300A IP, which supports up to 6 SIP connections and your land line. I'm using this with the 'robust' E49H handsets, not bombproof but a bit sturdier than your average cordless phone.
The Draytek and the Fritz also had support for fax lines, but we ditched fax about 10 years back.
The cost for an incoming SIP line ( and you can have local numbers or 0845/0870 types) range from free for a single number (sipgate.co.uk) to a few quid per month - much cheaper than BT rental. International calls are incredibly cheap. You can also get hard wired desk phones that plug into your Cat5 Ethernet connections. I've tried a few 'soft clients' on Android phones and they're good, but don't seem quite as reliable as the dedicated handsets.
Can't recommend it highly enough - we have our BT line, 2 UK local SIP numbers, a UK 0845 number. and a US number. Another provider is Zen internet (
http://www.zen.co.uk) we use them for our internet connection and they're great (With a genuine UK support desk!), I've not used their VOIP phone service but I believe it's competitive.
Edit: We also have an outgoing SIP connection via
http://www.localphone.com/services/voip which gives some of the best rates for calling UK mobiles, you can add an incoming number to this for ~£1 per month. You can also use localphone as 'calling card' type service which has its benefits. Clearly with all of these internet based services you need a reliable internet connection. Another thing I forgot to mention is that with most of these services you can get your voice mail/answer-phone sent direct to email which is really convenient for keeping track of communications. I also use Hullomail on my mobile for exactly this purpose.