The Justice Department has also sued to block the $39 billion merger, arguing it would stifle competition in the wireless industry.
AT&T said Sprint is just trying to protect its own interests.
"Despite their disingenuous rhetoric, Sprint is obviously afraid of competition," Wayne Watts, AT&T's general counsel, said. "Antitrust law is not about protecting a particular competitor, but rather is about protecting competition, and that is why we are confident that Sprint's complaint will ultimately fail."
If AT&T is allowed to acquire T-Mobile, it and Verizon will be the nation's largest wireless carriers, leaving Sprint a distant third.