What’s the best calling card for the UK if you’re going travelling around the world?

Me and a friend are going travelling later this year and we’re trying to work out what the best calling cards are to make phonecalls back to friends and family in the UK?

We’re going to be visiting north, central and south america, japan, china, south east asia, india and finland. So I’m trying to work out whether there are any good combined calling cards or whether we’re going to have to buy separates for each region/country.

Thanks,

Chris_A