ST. LOUIS – Telecommunications companies are beginning to shut down their 3G networks. For AT&T, that process started Tuesday.

What that means for consumers can be confusing. There’s a big change happening to cellular networks. Out with the old and in with the new.

“In order for 4G LTE to take off, they had to cannibalize the 2G spectrum they were using. For the 5G to take off, they have to cannibalize the 3G spectrum,” Sean Devereaux, director of customer-based sales for Tech Electronics, said.

AT&T is the first of the telecom companies to shut down its 3G network. It’s a costly, years-in-the-works process that will force people with older phones and devices to upgrade. Those that aren’t at least 4G compatible will no longer be able to send calls or text messages. T-Mobile will shutter its 3G systems on March 31. Verizon is scheduled to do it on December 31.

“AT&T is not going to flip the switch or Verizon isn’t going to flip the switch and all these devices turn to bricks, but they will start harvesting the spectrum. Today your alarm could communicate and tomorrow you’re going to be getting a call from our customer experience center saying we’re no longer receiving alarm communications from your unit,” Devereaux said.

These shutdowns were delayed multiple times over the last few years. Devereaux said the impact extends beyond cell phones and into fire alarms and security systems that will stop communicating with emergency services when 3G is gone. He said upgrading systems had been hampered by the pandemic and, now, supply chain issues.

“For the past two years, talking a customer into allowing you into their home or their business, or their long-term care facility or hospital, school, etc. It’s been ‘We don’t need you in here, we don’t want you in here.’ We can’t even get in some of these units fast enough for the people that are done procrastinating and now realize the urgency,” he said.

Devereaux said if you aren’t sure this affects you, it’s better to be safe than sorry.

“It’s important for everyone, especially for people who’re assuming a certain level of protection, to just double-check that with their service companies,” he said.

In a statement, AT&T told FOX 2, “For nearly two years, we’ve proactively sent numerous communications via direct mail, bill messaging, emails, and text messages to help customers transition to next-generation networks before 3G services end. We are working with them to make this process easier, including in a substantial majority of cases providing free replacement phones.”

Here’s a list of affected phones, devices, and security systems: