Found this on their website:
You can choose a subscription to call one country (your own or a different one), a region (for example, Europe, or US & Canada) or a selection of countries worldwide. And if you're calling a lot of different countries, you can buy a subscription for calls to each of them. A subscription works according to the country you’re calling to, not from, which means you can use it in any country – perfect for when you’re travelling.