The nation’s top telecom regulator is opening an investigation into CenturyLink after the company acknowledged it is struggling with a nationwide outage affecting emergency calling services in parts of Massachusetts, Texas and Washington state.

The multistate outage began Thursday night, with CenturyLink promising to restore service within a matter of hours. But on Friday morning, the company admitted it is still grappling with the issue, citing “additional technical problems” it discovered in the course of restoring service.

As the company worked to bring 911 support back online, dispatch centers were notifying the public to dial alternative, 10-digit numbers to report an emergency.

The cause of the disruption was not immediately clear on Friday. In a statement, CenturyLink reiterated Friday afternoon it is aware of the outages and that technicians are still at work restoring service.

“In case of an emergency, customers should use their wireless phones to call 911 or drive to their nearest fire station or emergency facility,” said spokeswoman Nikki Wheeler.

Other areas experiencing difficulties included parts of Arizona, Idaho and Wyoming, according to customer reports on the tracker DownDetector.com.

Calling the incident “unacceptable,” Federal Communications Commission Chairman Ajit Pai said Friday that the agency’s public safety staff will open a probe into the matter.

“The CenturyLink service outage is . . . completely unacceptable, and its breadth and duration are particularly troubling,” said Pai in a statement. “This inquiry will include an examination of the effect that CenturyLink’s outage appears to have had on other providers’ 911 services. I have also spoken with CenturyLink to underscore the urgency of restoring service immediately.”