NEW YORK — The head of the U.S. communications regulator said T-Mobile’s nationwide, hours-long outage Monday was “unacceptable” and that the Federal Communications Commission will investigate.

T-Mobile, one of the country’s three largest cellphone service providers, said it had a “voice and text wireless issue” that began around noon Monday. The company said at 1 a.m. Tuesday that all problems should be resolved.

The company blamed an internet-traffic issue that caused problems with its network for the outage.