Free International Calls from Mobile

Here's quite a clever trick that someone told me about. A company called Speakeasy is offering international calls via low rate UK numbers.
These numbers are within your voice minutes so if you have enough minutes you can call internationally for free. If you don't have the minutes then its 5p per minute - still pretty good for international.

The access numbers are:
For Orange, T-Mobile and Virgin - 07744 787501
For O2 and BT Landlines (5p per minute) -
0844 8255001

As I understand it the way this works is that these numbers have not been exempted from call tariffs by those networks. All the company does is have a switching system that allows you to dial on to the international number once our through. They make their money on the call rebate cost.