Disaster Message Dial is a voice message board that NTT provides when a disaster makes phone calls hard to get through. You dial "171" and use the phone number of the person whose safety you want to report as the key, then record or play back voice messages of up to 30 seconds each. Even while calls are congested, recording and playing back messages tends to get through relatively well.
Using it is straightforward: dial 171, press "1" to record or "2" to play back, then enter the key phone number (for a landline, starting with the area code). Numbers that can serve as the key include subscriber telephone, ISDN, Hikari Denwa, IP phone, mobile, and PHS numbers. Up to 20 messages can be recorded per phone number, but the actual number allowed and how long messages are kept are set by NTT to suit the disaster at hand and announced when the service opens. Calls placed from NTT East and NTT West phone services to record or play back messages are free of charge, and there is no fee for the message center itself. For calls from other carriers' lines or from mobile phones, the call charge is up to each carrier. Use from the disaster public phones installed at evacuation centers is free.
The service opens when a disaster such as an earthquake drives up calls checking on people's safety and calls into the affected area become hard to complete (network congestion). Both the decision to open it and the operating conditions, such as recording length and retention period, are set by NTT to fit the situation and announced through television, radio, and NTT's official website. Because the conditions change from one disaster to the next, it is safer not to assume the message count or retention period you saw in a previous activation will apply again.
So that nobody has to work it out in a panic, NTT sets aside times for practice use: the 1st and 15th of every 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). Practice use may be unavailable when a disaster is actually under way. During practice, the conditions are 30 seconds per message, 20 messages per phone number, and retention until the practice period ends. Messages you would rather others not hear can also be recorded and played back with a PIN, limiting them to specific people.
There is also a web version, the Disaster Message Board (web171), which lets you register and check safety information as text. Mobile carriers run their own disaster message boards as well. The pitfall is that unless your family has agreed on which phone number to use as the key, nobody will reach the messages you recorded. Sharing a rule such as "in a disaster, check Disaster Message Dial using our home landline number as the key" while things are calm makes it dependable. See the disaster phone guide for detailed instructions.