I've just moved to
http://www.firsttelecom.com/ as they specialise in cable phonelines.
I was with Onetel for years but their rates (and connection charge) just got silly.
Both use a box you put in series with your normal telephone line... no extra numbers to dial, etc... and you only pay for what you use after you used it.
My monthly bill used to around £4... the same calls will now cost around £2...