Earlier, AT&T, the No. 2 U.S. mobile provider, announced a promotion offering T-Mobile users a $200 credit to switch to its service, as well as a phone trade-in offer that could be worth another $250.
(Read more: Want $450? AT&T lures T-Mobile users with credit)
"This is a desperate move by AT&T on the heels of what must have been a terrible Q4 and holiday for them," Legere said in an emailed statement. "Consumers won't be fooled. … Nothing has changed—customers will still feel the same old pain that AT&T is famous for."
T-Mobile, the No. 4 mobile provider in the U.S., has been making headway against larger rivals by criticizing them in its marketing and by dubbing itself the "Un-Carrier."