what kind calling card should I buy

I am going to UK for 3 month.My family and my friends are in USA.I want to contact them.I think I should buy some international calling cards,Which can save money.There are many calling cards,which one should I choose.
You can buy calling card in UK with exclusive for USA that would be cheaper.There are many calling cards sale online.You should choose a right one.Maybe you will take a long time to compare.