Still using an analog cell phone? Better upgrade fast because on Feb. 19 those U.S. carriers still offering analog service will shut down their outdated networks.

Until this year, the Federal Communications Commission had required wireless carriers to maintain analog networks despite the fact that the numbers of subscribers using the aging service has been dwindling since the early 2000s.

If not for the FCC mandate, carriers would have probably turned off their analog signals several years ago, when the vast majority of the country switched to digital phones. Unlike analog phones, which transmit a direct representation of sound waves, digital devices convert sounds into 0’s and 1’s and thus can send and receive text messages and enable a host of other data-rich mobile features.

So why should anyone care about the analog shutdown? Well, in addition to several million mostly rural AT&T (T), Alltel and Verizon Wireless (VZ) customers, owners of some older cars will lose communication systems like OnStar when they go digital. (Sprint Nextel (S) and T-Mobile (DT), two relatively new carriers, don’t have analog networks). Wireless companies hope the shutdown will drive their analog customers to go digital but there is a risk that they will defect to other carriers or just let their service expire.

That’s why both AT&T and Verizon say they’ve been reaching out to those customers for more than a year, trying to lure them into digital plans with a combination of calls and letters.

“Over the past year or so we’ve been reaching out to those customers and encouraging them to upgrade and even offering them free handsets,” says Verizon spokeswoman Debra Lewis. “Hopefully people won’t be caught without service come Feb. 19.”

Since many consumers don’t even know whether their current handset is analog or digital, the FCC has issued an advisory with some tips on how to tell which one you have: For example, if your wireless phone uses a SIM card or has advanced features like text or instant messaging, it is digital.

Still, it’s likely some unsuspecting customers will be unpleasantly surprised when they wake up on Feb. 19 and realize their phone has become a true brick. But the move is a necessary step for the wireless industry, which will now be able to use the small portion of spectrum that will become available for more lucrative digital services.

Verizon says that the amount of analog traffic on its network is well under 0.5 percent and has been declining over the past few years. According to research firm M:Metrics, those who will be affected by the shutdown of analog networks number fewer than four million out of more than 250 million total wireless subscribers in the United States. Most importantly, none of those analog customers are eat up a lot of minutes or subscribe to data plans. In other words, they’re not big moneymakers for the carriers.

“The majority of folks with these phones are 65 plus and are the ‘keep your phone in the glove box’ kind of people,” says Jaimee Minney, a director at M:Metrics.