Vodafone (UK) are awesome within Europe - when I am abroad I pay £2.50/day to use my normal price plan abroad (so I still get unlimited minutes/texts, and 6GB data, for no cost other than that daily fee). For calling European numbers they aren't quite as good, but for £5/month the prices drop from £1/min to between 5p and 20p/min depending on the type of number I'm calling.
Outside Europe they're still awful though, something like £1.50/min to call to America/Asia from UK, and same for both placing and receiving calls when in those continents.
Last time I was in America I picked up a cheap phone at a supermarket that gave me 1c/min phonecalls to anywhere in the US or even abroad - including calling UK mobile phones. That saved me a lot of money.