If you have a BT line and use call 18866 all calls to UK landlines (but not 0845 etc.) are free (but there might be a 1p connection charge!!)so it is definitely cheaper

I also find that calls abroad are pretty cheap, 2p a minute to France and 3p a minute to Cyprus. When my BT line was faulty it worked well with my NTL line but I had to dial another number first so whatever you have I'd recommend giving it a go.