By J. D. BIERSDORFER

Q. If you have an AT&T mobile phone with no landline and the service won’t connect to 911, what should you do in the case of a medical emergency? I have a 4-year-old and worry about such things.

A. Planning ahead can help save time in an emergency in the event that your mobile carrier is unable to connect your call to the 911 dispatcher. Immediate busy signals or calls that ring but never get picked up are two clues that your service provider is suffering a 911 failure.

Glitches in the system are infrequent but do happen, so before they do, look up the emergency 10-digit direct numbers of your local police precinct, fire department, hospital or medical service and program them into your phone’s contact list. (Putting 911 and other emergency numbers in your speed-dial favorites may not be the best idea, however, as accidentally dialing them can place unnecessary calls and tie up the line.)

For further reading, the Federal Communication Commission’s website has detailed information about the 911 emergency response system.

You should be able to find these numbers in the local telephone directory for municipal services or on your town or city’s local government website. (For those curious about the general process, New York City’s site includes an informational graphic showing what happens when you do call 911.) As you would when calling 911 on a cellphone, give the dispatcher your exact location and phone number as soon as you connect.

If a 911 failure does occur, your carrier may post the 10-digit numbers for fire and police departments on its site or on Twitter. Municipal websites in the affected areas may also post the numbers online. Some towns and cities that use the 311 number for nonemergency services may direct callers to use that number instead of 911 if the wireless carrier can connect to it.

The 911 system was put into place before mobile phones became widespread, and government organizations are working to update emergency call handling and location tracking. Congress has a NextGen 9-1-1 Caucus, which is working to improve and enhance emergency response systems.

The ability to send text messages to 911 centers (a function that would help those with hearing impairments as well as those who cannot make voice calls) is available in limited areas. The Federal Communications Commission’s website has information on text-to-911 services.

Mobile phone manufacturers may also take their own steps in the future. At least one Android hardware maker is working with a company called LaaSer to provide cloud-based technology that lets cellphones automatically share accurate location information with 911 call centers.