It might be worth looking at your Telephone/Mobile provider, some of them do addon's that vastly reduce the rates of international calling.
Or
Try looking at some of the SIP/VOIP providers?
Here's a few I can think of.
http://voipfone.co.uk
http://sipgate.co.uk
http://aaisp.net/telecoms.html
http://gradwell.com
http://voiptalk.org
I'm sure there are lots more, some will be cheaper than others. Most will support either a softphone or a hardware device such as an IP phone.