Of course you could ditch the line rental completely and use VOIP. I have been using it for two years now and its never let me down. With free calls to virtually everywhere in the world (which I take great advantage of) as well as the UK. Plus all the usual phone add ons....Voice mail, 1471, call waiting, 999 capability...etc etc etc. All for £7.50 pm No line rental. If you want it cheaper than that you can get it with a few less extras from various companies....
I'll never go back to BT