Here's an interesting site for those who like to work the system. Basically someone took the time and the inclination to show you how you can utilise all the FREE calls to international landlines offered throughout the various fineara sites.

http://www.myvoipprovider.com/Free_VoIP/

Be advised that the offers change on a frequent basis. This site was an updated of 22 Feb 2006.

It sure beats sitting there with a pen and paper trying to do it yourself. Interesting that it shows the minimum credit in euros. If you work out the euro rate to the pound, it's cheaper to top up in euro.
Voipcheap = 5 euro.(=£3.50)  Their english site =£10