I have just come across this page from BT (see below) and it seems if you are with BT and have an Iphone you can use it to call from anywhere in the world and get charged the normal cost as if you are phoning from you landline. If you have an inclusive bundles package the calls are included which means they could be free. All you have to do is download the BT SmartTalk app from the itunes store. The calls are best made over WIFI otherwise you pay data charges to your mobile company.
This is the link to the BT page explaining it in full.
PS. It seems it is also available for Android phones from the Google Play website.
This is the link to the BT page explaining it in full.
PS. It seems it is also available for Android phones from the Google Play website.