Disaster Message Dial (171) is a safety confirmation service provided by NTT during major disasters. Using a disaster-area phone number as the key, voice messages of up to 30 seconds can be recorded and played back. NTT starts the service at its own discretion once calls into a disaster area become hard to connect, and it is easy to remember through the Japanese wordplay "i-na-i" (171, meaning "not home").
Usage is simple. After dialing 171, press 1 to record or 2 to play back, then enter the disaster-area phone number (home landline or mobile number). The number of messages that can be stored for one phone number is set for each disaster within a range of 1 to 20, and is announced when the service starts. Messages remain available until that round of the service ends. It can be used from landlines, mobile phones, and public phones. Calls placed from NTT East and NTT West phone services to record or play back messages are free of charge, while the charge for calls from other carriers' lines or from mobile phones is up to each carrier. The web version "Disaster Message Board (web171)" also allows text message registration and checking, which may be more convenient from smartphones.
The service exists because of how calls pile up during a disaster. When a major disaster strikes, calls checking on the safety of family and friends concentrate into a short span, far exceeding the volume the network equipment can handle. Carriers may restrict outgoing calls to keep that equipment from being overwhelmed, and while those restrictions are in place ordinary calls become hard to connect. The Disaster Message Dial narrows its purpose to recording and playing back short messages, and is offered as a way to exchange safety information even while ordinary calls are being held back. The message dial itself can also become congested, however, so it is safer to treat it as something to retry after a while rather than as a line that will always connect.
Advance preparation makes the difference. Trial use is available on set dates: the 1st and 15th of each month (00:00 to 24:00), the first three days of January (1 January 00:00 to 3 January 24:00), Disaster Prevention Week (30 August 09:00 to 5 September 17:00), and Disaster Prevention and Volunteer Week (15 January 09:00 to 21 January 17:00), so you can practice the actual operation with your family. Deciding in advance "whose phone number to use as the key" matters most. If all family members use the same number, such as the home landline, safety confirmation is completed simply by recording and playing messages for that number. If you no longer have a landline, pick one representative mobile number as the key and share it with everyone. Along with emergency numbers, review usage in Emergency Numbers in Japan (110, 119, 118) - A Complete Guide. For the wider picture of calling during a disaster, see How to Make Phone Calls During Disasters - Using the 171 Message Dial.