A robocall is a phone call mass-dialed to large numbers of phone numbers using an auto-dialer system, playing pre-recorded voice messages. A single system can place large volumes of calls in a short time, which is why robocalls are used both for legitimate purposes and for abuse.
Legitimate uses include municipal disaster evacuation alerts, medical appointment reminders, school emergency contact networks, political party announcements during election periods, and bank fraud alerts. These are made with prior recipient consent or based on public interest. Conversely, an automated message that asks for money, personal information, or a button press, when you can think of no consent you gave and no public-interest reason for the call, is safest treated as falling outside these legitimate uses.
However, abuse for fraud and illegal sales activities has drawn countermeasures in many countries. Typical techniques include recorded messages creating anxiety with "You have unpaid fees," "Your account will be frozen," or "Important notice from the tax office," then transferring to an operator via button press to extract personal information or money. Some are combined with caller ID spoofing so that a bank or government number appears on the display. In Japan, telemarketing calls fall under the Specified Commercial Transactions Act, which requires a business to state its name and the purpose of the call before soliciting (Article 16) and prohibits further solicitation of a consumer who has declined (Article 17).
When receiving a robocall, the response is to share no personal information once a recorded message plays and hang up immediately. Do not follow instructions like "Press 1 to connect to an operator" or "Press 9 to stop delivery." Pressing a button lets the sender log your number as one that answered, which can keep it on their calling list. Registering the number for call blocking and enabling spam call filters will stop repeat calls from that number, but a call from a different number can still get through, so blocking number by number is not a complete safeguard. As a countermeasure against robocalls that rely on number spoofing, there is STIR/SHAKEN, a framework in which carriers verify that a calling number is genuine. See robocall detection tips for how to recognize such calls and what to do when one arrives.