AT&T Inc. is shutting down its second-generation, or 2G, wireless networks by 2017 as it continues to upgrade its systems to faster technology and better use its limited airwaves.

The telecom giant said about 12% of its contract wireless customers, or roughly 8.4 million people, were using 2G handsets at the end of June, but it will work "proactively" in coming years to move them to more advanced devices. Like the other major carriers, AT&T's customers mostly use phones with 3G, or third-generation, technology, and it is aggressively...