Originally Posted by CrashandDie

Actually, the "+" is a shortcut for international calls, however, they are not always equivalent to "00".

If my memory serves me well, the US & Canada used "011" to dial internationally, where most of Europe dialed "00". Similarly, Kazakhstan users dial "8", wait a second, and then "10" in order to be able to dial international numbers (I'll let you imagine how well that works on mobile devices! "But wait, there's no dial tone mate!")

The + sign simply standardises the international prefix.