Wangiri is the act of ringing a phone just once and hanging up. The purpose is to leave a number in the recipient's call history and prompt them to call back. The Japanese term "wangiri" (one-ring cut) is used internationally, even appearing in ITU (International Telecommunication Union) reports.
Wangiri serves three main purposes. First, fraud that lures callbacks to expensive premium rate numbers (one-ring scam). International number wangiri is particularly dangerous - calling back can incur charges of several hundred yen per minute, with automated messages or hold music designed to extend the call. Second, building marketing lists - numbers that call back are recorded as "live numbers" (active numbers) and added to telemarketing target lists. Third, simple harassment or phone harassment.
Wangiri and silent calls are often confused but differ in nature. Wangiri disconnects after 1-2 rings with the goal of not being answered. Silent calls involve silence after the call is answered, caused by predictive dialer malfunctions or harassment. Wangiri aims to "get a callback," while silent calls are "silence after answering."
The countermeasure is clear: do not call back unknown numbers from wangiri, and use phone number lookup services to check the source. Wangiri from international numbers starting with + is very likely fraud - the best response is to ignore and delete. You can also use your smartphone's call blocking settings to block incoming calls from specific country codes. See One-Ring Scam Dangers for the latest tactics and How to Block Nuisance Calls for setup instructions.