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.
