DALLAS — AT&T Inc. will start selling the previous-generation iPhone — the iPhone 3GS — for $49 starting Friday to customers who buy the smart phone with a two-year service contract and data plan.

The wireless company, which is the exclusive U.S. carrier for the iPhone, made the announcement Thursday. AT&T said it will include iPhone 3GS devices sold through AT&T and Apple.

The iPhone 3GS was released by Cupertino, Calif.-based Apple Inc. in 2009 and had been sold for $99 by AT&T and Apple since the release of the iPhone 4 last June. It can use the same iOS4 software as the iPhone 4.

AT&T, which is based in Dallas, is currently selling refurbished iPhone 3GS’ on its website for as low as $49.