Country calling codes are numeric prefixes used to route telephone calls between different countries. They are assigned by the International Telecommunication Union (ITU) under standard E.164. The '+' symbol in a phone number technically represents the International Direct Dialing (IDD) prefix, commonly known as an exit code.
When you dial a number starting with a country code, the telephone network understands that the call must be routed out of your current national network. The exact exit code varies—in the US it is '011', while in most of Europe it is '00'. The '+' symbol automatically translates to the correct exit code for the network you are calling from.
The logic behind the ITU numbering zones
The ITU divided the world into 9 geographic zones, assigning the first digit of the country code based on these zones. Zone 1 covers the North American Numbering Plan (US, Canada, and parts of the Caribbean), which is why they all share the '+1' country code.
Zone 3 and 4 cover Europe (e.g., +33 for France, +44 for the UK). Zone 8 covers East Asia (+81 for Japan, +86 for China). The number of digits in a country code varies from one (like +1 or +7 for Russia) to three (like +353 for Ireland), usually depending on the population and telephone density when the codes were assigned.
Dropping the trunk prefix (the leading zero)
A common source of confusion when dialing internationally is the 'trunk prefix', which is usually a leading '0' used for domestic long-distance calls in many countries. For example, a London number might be written locally as 020 7946 0000.
When calling this number internationally, the domestic trunk prefix (0) must be dropped. You dial the exit code or plus sign, the UK country code (44), the area code without the zero (20), and the local number. The correct international format is +44 20 7946 0000. Formatting numbers to the E.164 standard ensures they can be dialed globally without error.