In a limited-time offer, T-Mobile customers who switch to AT&T will receive a $200 credit per line, which includes family plan customers.
The per-line credit could be on top of another credit of up to $250 if the customer trades in their current smartphone. However, trade-in values vary based on the model and age of the smartphone.
T-Mobile has already posted two quarters of subscriber growth as a result of the promotions.
AT&T, No.2 U.S. mobile provider, is vulnerable to T-Mobile's aggressive promotions because both companies use the same network technology making it easier for their consumers to switch.