A country code is the number that identifies the destination country when you place an international call. The ITU (International Telecommunication Union) assigns these codes to countries and territories, and Japan's is +81. In E.164 format, the leading 0 of the area code is dropped after the country code (for example, 03-1234-5678 becomes +81-3-1234-5678). Mobile numbers follow the same rule: 090-1234-5678 is written +81-90-1234-5678. Leave that leading 0 in place and calls may fail to connect or verification texts may never arrive, which makes it the single most common stumbling block in practice.
Country codes are grouped into geographic zones. Zone 1 is North America (United States and Canada: +1), Zone 2 is mainly Africa (+2xx), Zones 3 and 4 are Europe (United Kingdom: +44, France: +33, Germany: +49), Zone 5 is Latin America (Brazil: +55), Zone 6 is Southeast Asia and Oceania (Australia: +61), Zone 7 is Russia and nearby countries (+7), Zone 8 is East Asia (China: +86, South Korea: +82, Japan: +81), and Zone 9 is West and South Asia (India: +91). Codes run from one to three digits, and the first digit is the zone. In some cases the zone digit is the whole country code, as with +1 and +7, and +1 is shared by several countries and territories across North America, and +7 likewise serves more than one country. A country code alone, in other words, does not always pin down where a call originated.
From a nuisance call standpoint, an incoming call from an unfamiliar country code may be a one-ring scam. Calls from Zone 2 African countries (Guinea +224, Sierra Leone +232, Liberia +231) and Zone 6 Pacific island nations (Nauru +674, Tuvalu +688) deserve particular caution, and these country codes have been reported in such calls. How the scheme abuses the way international call revenue is shared, known as International Revenue Share Fraud (IRSF), is explained in the one-ring scam entry. Calling back puts international rates on your bill, so unless you have contacts or business partners overseas, the safe move is never to return a call from an unfamiliar number beginning with +.
You will need Japan's +81 when calling Japan from abroad and when registering a phone number in international format. Sign-up forms for web services sometimes require the international form (+81). Forms differ in how they treat hyphens and the + sign, and when the country code sits in its own selector, take care not to leave the leading 0 in the number field. Understanding the rules of phone number formatting makes converting between domestic and international forms straightforward. The Japan country code guide covers how to use +81, and international phone scam techniques covers how to handle suspicious calls.