In Asda you will get the call-time card for free. It is loaded with £0.50 for you to try out. Calls to international MOBILES are the cheapest I could find. For example Poland, India and many other countries are charged at £0.01
You would call the access number 0330... which will use your inclusive minutes on contract mobiles / landlines.
AND another option is to use the 0800... access number which suppose to charge extra 1.5p per minute BUT in fact it does not charge anything. It is 0800 and it is free, not even itemised on my Plusnet bill. I called several hours last month and was charged nothing.
Remember, the card is free and the first £0.50 (50 minutes) is free for you to try!