This blooming number keeps calling, and as soon as I answer it hangs up. I want to phone back and give them a piece of my mind, they've been calling for days and days and almost always when ds is asleep and it is P!ING ME OFF. But I don't want to call if it's some high charging crap that's hoping you'll call back to get them lots of money! 
Is there a way you can find out whose number it is without calling? Or ask BT to block the number? TIA