NEW YORK (AP) — As telecom companies rev up the newest generation of mobile service, called 5G, they're shutting down old networks — a costly, years-in-the-works process that's now prompting calls for a delay because many products out there still rely on the old standard, 3G.

AT&T is scheduled to be the first carrier to shut down its 3G network on Feb. 22. T-Mobile will shut down its 3G network by summer and Verizon in December.

The home-alarm industry has asked the Federal Communications Commission, the U.S. regulator, to delay AT&T's network sunset until December. The FCC is monitoring the 3G phase-out and working to "implement safeguards" for older phones and other devices, spokesperson Paloma Perez said late Monday.

Verizon has already pushed back its shutdown — twice — from an original target date in 2019, saying customers needed more time to update their devices. T-Mobile has also delayed the shutdown of the Sprint 3G network it acquired in 2020, to the end of March; it'll shut down the T-Mobile 3G network by July 1.

WHY IS 3G SHUTTING DOWN?

First, some history. AT&T's 3G network launched in the U.S. in 2004; later that decade it was the exclusive carrier for early iPhones, helping usher in the first phase of the smartphone era. The networks we currently rely on for video streaming, social apps, Uber and other must-haves of the modern era mostly use the subsequent 4G standard.

For the carriers, shutting down 3G is an efficiency move. As they upgrade to the latest technology, they shut off outdated networks and use the freed-up bandwidth for newer — and what they hope will be more profitable — services.

WHAT IF I HAVE AN OLDER PHONE?

People with older phones that aren't compatible with 4G will have to upgrade; once 3G shuts down, those phones won't work for calls or texting. AT&T says it has reached out to offer its customers free replacements via letters, emails and texts. Spokesperson Jim Greer said fewer than 1% of AT&T's consumer devices, including phones, tablets and watches, will lose cellular service, but declined to say how many devices that is. The company reported about 196 million phones and connected devices using its network in the most recent quarter.

WHAT ABOUT OTHER DEVICES?

Industry groups have also raised concerns about other products that will need to be replaced or updated — everything from home fire alarms to ankle bracelets used by law enforcement. It's not certain how many outdated products are out there, or how big a deal it would be if updates take place after Feb. 22.

The alarm industry and other companies say they have had issues replacing devices even though they've known about the coming shutdown for years. Recent setbacks include both pandemic-triggered supply-chain issues and customers hesitant to let technicians into their homes during the pandemic.

HOW MANY OUTDATED PRODUCTS ARE OUT THERE?

It's not really clear. An alarm-industry lobbying group estimates that 1.5 million customers still need to upgrade their fire or burglar alarms, while about half a million have medical alert devices that run on 3G; it said most rely on AT&T service. While an unnetworked fire alarm will still sound an alarm if there's smoke, it won't be able to contact the fire department. Likewise, burglar alarms won't route to emergency responders if triggered. Not all providers say there's an issue. ADT said in November that it was on track to update its AT&T customers by February; a spokesperson declined to offer an update on Sunday.

AARP, the advocacy group for adults over 50, is also concerned that users of medical alert systems — those necklaces and bracelets, like Life Alert — that connect users to emergency call centers won't know their gadgets don't work anymore or won't be able to replace them in time.

HOW SHOULD I PREPARE?

Check your phone to make sure it will still work. Here's an AT&T list of devices that the carrier says will work normally after Feb. 22. Call the companies that make or service your burglar and fire alarms and personal medical alert systems to see if any need an update. If so, schedule a service visit immediately or get a new device shipped.

OK, DONE. ANYTHING ELSE I SHOULD WORRY ABOUT?

A few. One manufacturer of ankle bracelets for people on probation, parole or pre-trial release said it hasn't been able to update many 3G-reliant devices. A premature shutdown could potentially allow tens of thousands of offenders like child abusers, sex offenders and drunk drivers to go unmonitored while out of prison, Alcohol Monitoring Systems wrote in an August FCC filing. The company did not respond to questions.

Zonar, which provides GPS and other services for buses and trucks, says tens of thousands of vehicles will be affected. Trucks that aren't upgraded may have to be idled if drivers can't electronically log their hours as federal rules require. Zonar has a workaround for the trucking industry, but not all customers have placed orders for it, said Susan Corscadden, a company marketing executive.

Affected school districts could also lose their ability to track the location of school buses, while their drivers may not be able to use GPS systems for directions. The National Association for Pupil Transportation seconded these concerns in a September filing, although Noelle Ellerson Ng, the group's legislative liaison, said in an interview that she hasn't heard school superintendents raise the issue.

SO DOES THE SHUTDOWN POSE A SERIOUS PUBLIC-SAFETY THREAT?

It's unclear. "There's a lot of uncertainty about the impact and about how many people are affected," said Tom Kamber, the executive director of Older Adults Technology Services, a nonprofit affiliated with AARP. The AARP has asked the FCC to delay the AT&T shutdown until December.

Public Knowledge, a public-interest group, also urged the FCC to block the February shutdown unless AT&T can show that it has made sure that essential services won't be disrupted or that it can restore service immediately if problems arise.

SO WHY NOT DELAY?

AT&T says that delaying the shutdown would hurt its 5G rollout, degrading its customers' ability to use their service and causing more dropped calls. The company argues that the alarm companies have had years to upgrade their customers' devices. The company also says the FCC doesn't have the authority to stop its shutdown.