ATT Inc on Friday offered customers of No. 4 U.S. mobile provider T-Mobile US Inc a $200 credit to switch to its service on top of a phone trade-in that could be worth another $250, amping up competition between the two rivals. ATT, the No. 2 U.S. mobile provider, announced the promotion after T-Mobile spent months appealing to ATT customers and in anticipation of a new competitive offer expected from the smaller company next week.