Today both AT&T and Verizon shut off their analog networks based on an FCC decision way back in 2002 tto "analog sunset" Advanced Mobile Phone System (AMPS) networks. AMPS, first generation cellular technology developed in the early 1980's, requires separate frequency channels for each phone conversation and is extremely bandwidth hungry.
Both Verizon and AT&T gave up front notification and worked with analog customers for years to get them switched over (almost all were years ago) so the shut down will have negligible effect. There may still be a few AMPS networks out there in this country after today - shutdown is optional and some small rural carriers may have not shut down today. Eventually they all will.
You may have read around the first of the year about General Motor's OnStar systems and how the OnStar network was converting to CDMA based communications on January 1, 2008. Here's a quote from InfoWorld on the InStar conversaion:
Some users of wireless roadside assistance have also been left behind in the transition......... The automaker didn't wait for the Feb. 18 deadline but instead shut down its analog service on Jan. 1. In a statement on the transition last year, GM said about 90 percent of its subscribers' cars had CDMA or could be converted to use it. Others would lose their OnStar service. The wholly owned subsidiary of GM said last October it had about 5 million subscribers.
Residential and business alarm systems have been preparing for the shut down for a while also. Here's more from the InfoWorld piece:
....... AMPS isn't only used for cell phones. Many alarm companies use the system to alert police or fire departments to emergencies at homes or businesses. About three years ago, the Alarm Industry Communications Committee (AICC) industry group took a survey which revealed that just under 1 million of the approximately 30 million monitored home and business alarm systems used an analog cellular network, said AICC chairman Louis Fiore. About 850,000 of them used the system only as a backup in case the phone line was cut, he said.
In the end, faster and more efficient digital systems took over with AMPS becoming too expensive to support and maintain.