Jim
I am using VOIP Cheap
http://www.voipcheap.co.uk/en/index.html which is similar to Skype.
You have to initially pay £10 to set up your account, but dont worry you dont lose that, any calls that are chargeable which are mainly to mobiles or premium rate lines such as 0845 and the like (and very cheap rates they are too) are deducted from this £10 which you keep topping it up.
The nice thing with VOIP Cheap is that you can phone normal landlines. UK landline calls are free, excepting to usual premium rate lines, but also calls to landlines in the rest of europe and the states are also free.
The other bonus is you are given a telephone number, which you can give to people, and if you use the divert feature to divert any incoming calls to your landline, you done even have to have your computer on.
Hope this is of help.
Mike