Hmm... that sounds like Skype on the N900 can't be used to call phone numbers at all then. Because you're always supposed to use that format, even in your own country, as Skype is "outside" of the local phone network to start with. Hopefully someone can clarify this. You're certainly not meant to use e.g. '00' or some such, because as someone explained earlier this isn't universal for all countries, while '+' is.