I pay around 15 GPB/month for Vonage. This gets me unlimited calls to North America, as well as very low-priced calls (~2p/min) to most other countries. It gives me a local, UK phone number with free voicemail (something BT would charge for on our landline) and all that good stuff. It also gives me a virtual number in the U.S. (I was able to choose a number in my hometown) which allows my family to call me.
Being able to just pick up the phone and call my sister or best friend whenever something stupid pops into my head, and for them to be able to do the same, makes it the best 15 quid I could spend, hands down.
There may be slightly cheaper options available, but for convenience, reliability and call quality, I can't recommend Vonage highly enough.