In a regulatory filing, the Dallas-based company said 12 percent of its customers on contract-based plans, or 8.4 million people, have 2G phones.
AT&T said it's shutting down the older network, which doesn't support high data speeds, city by city. It said earlier this year that the process has started in New York City, and it's trying to move the city's 2G subscribers to new phones.
By shutting down 2G and using the same space on the airwaves for 4G, AT&T can increase data capacity by more than a hundred-fold. Data use is skyrocketing as people adopt smartphones, and the company is facing a "spectrum crunch" in some areas.
Other companies are also "refarming" 2G spectrum. Sprint Nextel is shutting down the Nextel 2G network and moving subscribers to Sprint 3G.
The second generation of wireless technology debuted in the 90s and was the first one to apply digital technology to cellphones, boosting call quality and network capacity. Third-generation networks, with higher data speeds, starting becoming widespread in the U.S. about six years ago.
Among the 2G devices on AT&T's network are the first Apple iPhone, which debuted in 2007.