After months of aggressive moves by T-Mobile US to lure customers from other carriers, No. 2 operator ATT Inc counterattacked on January 3 by offering to pay consumers to switch from T-Mobile. On Wednesday T-Mobile upped the ante, saying it would pay hefty exit costs for converts. The moves by Sprint and ATT come after No. 4 U.S. operator T-Mobile, a long-time industry straggler, was able to report three full quarters of customer growth after four years of losses. Investors had hoped ATT and market leader Verizon Wireless would be able to shrug off T-Mobiles moves, since they already control about two-thirds of the market.