From Wiki

Martin Lewis' Money Saving Expert website provides a link to a Call Checker that enables you to look up the cheapest international long distance phone services in the UK. Some of them can even be used from mobile phones.

A free solution, although it does tie you to your computer, is Skype. You can use it for text messaging, speaking to someone on another computer through a headset, or seeing and conversing with someone on another computer if you both have webcams. The Skype software and service itself is free, as long as one computer is communicating with another. If the other party does not have Skype software, you can use Skype to make an international long distance call to a regular phone for the price of a local call in the destination country.