A new year has arrived, and ATTs resolution seems to be kicking the competition. Starting today (Jan. 3), the carrier is offering up to $450 per line for T-Mobile customers who make the switch and trade in an eligible smartphone. T-Mobile customers will receive a $200 credit per line if they transfer to an ATT Next plan, buy a new device at full value or activate an existing smartphone with ATT.T-Mobile users can then trade-in their current handset for a promotion card of up to $250, which can be used on ATT accessories and services.