The cash and stock deal also reduces the number of national wireless carriers from four to three.
The other two are Verizon Wireless and Sprint Nextel Corp.
The acquisition would give AT&T 129 million subscribers, vaulting it past Verizon's 102 million. The combined company would serve about 43 percent of U.S. cellphones.
Customers aren't expected to see any changes for awhile, as the deal could take a year to close.