Well in my experiance, I had bad trouble when I was using a british sim in spain and my friends from the uk where calling me we were both getting charged so I invested in a spanish sim card and phone and it was the best thing I ever did, when my mum calls me from the uk when I am in spain she pays to call me and I dont pay anything but I'm not sure how true this is for everyone else but I have never been charged when I have been away and someone from the uk calls me.

Hope this helps

Emma