For more than 12 hours on Monday, T-Mobile users were unable to send voice texts or make calls, including to 911.
Now the FCC is investigating.
T-Mobile is the third largest provider in the U.S. Behind AT&T and Verizon.
It blamed an internet traffic issue for the problem but the other two networks say they operated normally on Monday.
Public safety officials had to warn users to have an alternate plan if they needed to reach 911.
The FCC is calling it unacceptable.
It has fined T-Mobile in the past for nationwide outages.
