Country code, national code, subscriber number: the structure behind a number written with a plus.
The three parts
- Country code — one to three digits: 7 for Russia and Kazakhstan, 39 Italy, 90 Turkey, 1 for the USA and Canada.
- National code — a city code (495, 812) or a mobile code (900, 916) — kept in the number, but without a leading zero.
- Subscriber number — everything else.
Exit codes
Calling from a fixed line usually requires an international prefix: 810 in Russia, 00 in most of Europe, 011 in the USA. From a mobile in roaming you do not need one — the plus sign does the job.
Why the plus is universal
A number stored as +7 495 123-45-67 works from any country, in roaming and in messengers. A number stored with a national prefix works only at home.
The leading zero trap
City codes are written with a leading zero inside the country (06 for Rome) and without it internationally (+39 6). Keeping the zero produces an invalid number.
Length matters
E.164 allows up to 15 digits. If a number is longer, something was duplicated; if shorter, a digit was dropped.
See also: why Russian numbers start with 8, how to call any country.