Figuring out exactly how to dial a phone number in one country while in another country (particularly if neither one of them is a country with which you have much telephonic experience) can be a huge pain in the ass.
Just a few minutes ago, while making sure I was telling someone in Japan the correct way to reach my apartment line, I stumbled across this very handy website. It lets you specify a dialing and target location and very conveniently breaks down the entire dialing sequence into whatever international, national, or local dialing prefixes apply.