An international call is a phone call made between different countries. When calling from Japan, you dial the international access code 010 + country code + the recipient's phone number (omitting the leading 0). For example, to call New York (country code 1) at 212-555-1234, you dial 010-1-212-555-1234. From mobile phones, you can also long-press "+" and enter the country code directly. Forgetting to drop that leading 0 is the most common stumbling block, because the call simply fails to connect.
Fee structures vary significantly depending on the calling method. Traditional phone line services (NTT, KDDI, SoftBank international calling) set their per-minute rates by destination, so the same one-minute call costs a different amount depending on which country you reach. Those rates are revised from time to time, so checking your carrier's rate table for the destination before you dial is the surest way to avoid an unexpected bill. Mobile unlimited calling plans generally cover domestic calls, and whether international calls are included depends on the individual contract. If they are not included and you keep calling without noticing, the bill can end up far higher than you assumed. VoIP services such as LINE and WhatsApp, by contrast, carry voice over the internet, so no international telephone network charges apply; they consume mobile data instead. The trade-offs are that the other party has to use the same app, and that call quality depends on network conditions.
International call-related fraud also requires caution. In the international version of one-ring scams, the phone rings once and then cuts off, prompting a callback from the missed-call log. Country codes in Africa and the Pacific Islands have been reported in such cases. Calling back connects to a premium-rate number that bills you for an international call, and even a short conversation can cost an order of magnitude more than a domestic one. Unless you have friends or business contacts overseas, the safe rule is not to return calls from unfamiliar numbers beginning with +. Additionally, caller ID spoofing can make international calls appear as domestic numbers, so judging safety by the displayed number alone is dangerous.
For incoming calls to Japan (calling Japan from abroad), dial Japan's country code +81 followed by the number with the leading 0 removed (e.g., 03-1234-5678 becomes +81-3-1234-5678). Managing numbers in E.164 format makes number conversion smooth for international calls. See the reported techniques in international phone scam techniques and calling details in the Japan country code guide.