(Reuters) – The Federal Communications Commission (FCC) will probe an extensive T-Mobile network outage that impacted customers across the United States, the head of the U.S. telecommunications regulatory agency said on Monday.

“The T-Mobile network outage is unacceptable. The @FCC is launching an investigation. We’re demanding answers – and so are American consumers,” FCC Chairman Ajit Pai said on Twitter.

Neville Ray, president of technology at T-Mobile, said on Twitter Monday that engineers were working to resolve a voice and data issue that has affected customers around the country.

He said later that data services were now available and some calls were completing. “Alternate services like WhatsApp, Signal, iMessage, Facetime etc. are available,” he added.

T-Mobile US had 86 million customers at the end of 2019. T-Mobile did not immediately respond to a request for comment on the outage.

In 2018, Pai backed the merger of T-Mobile and Sprint Corp saying it would lead to improved 5G coverage in the United States and would bring much faster mobile broadband to rural Americans.

T-Mobile on April 1 officially completed its $23 billion merger with Sprint, solidifying its position as the No.3 wireless providers in the United States.