An example, that just happened to me, for which I feel kind of dumb about, so here: Enjoy my shame.
In the UK, much like the US, we have area codes, and then the actual phone number. So my home number would be something like 01277 123456 (obviously not my real number, but the area code is real). When you are dialling locally, like in the US, you don't need to dial the area code. When you dial another area code you just dial the other area code... say 01368.
In the US you need to preface that "long distance" area code with a 1, so SF's code is 415, but to call it from Oakland I'd call 1-415-whatever.
Which as you can guess totally threw me for a loop. I wondered why I was getting some annoyed latino guy. The reason it is so wierd to me is that its basically like the UK using the country code for every call. We do something similar (all area codes include the 0 at the begining for this reason and you drop it for international calls) but not using the country code.
So yes; subtle, small, differences... but sometimes important ones.