AT&T Inc., the second-largest US mobile-phone carrier, is cutting by half the price of the Apple Inc. iPhone 3GS, making way for a lineup of handsets designed for its faster network due later this year.

The price cut, to $49 from $99 for users on contracts, takes effect tomorrow, Mark Siegel, a spokesman for Dallas-based AT&T, said.

AT&T said yesterday that it plans to introduce 20 devices this year for its next-generation mobile network.

The new products are aimed at helping AT&T catch rivals Verizon Wireless and Sprint Nextel Corp. in starting so-called 4G service. AT&T is also eager to keep customers after it loses its status as the sole US provider of Apple’s iPhone.