AT&T, a US telecom service provider has restored its network after it went down for about ten or 11 hours leaving its customers unable to place calls, text or access the internet.

In a statement, AT&T said its service has been restored after a system fault was identified.

“Based on our initial review, we believe that today’s outage was caused by the application and execution of an incorrect process used as we were expanding our network, not a cyber attack. We are continuing our assessment of today’s outage to ensure we keep delivering the service that our customers deserve.

“We have restored wireless service to all our affected customers. We sincerely apologize to them,” AT&T said in a statement. The company added that it is “taking steps to ensure our customers do not experience this again in the future.”

The company denies any external hack or cyber attack but argues the outage may have been caused by an internal error. Many users were advised to use Wi-Fi to call and send texts as the Federal Communications Commission and other federal agencies continued to investigate the matter.

Some Verizon and T-Mobile customers reported a bit of network outage but it was far less widespread but T-Mobile and Verizon argued their networks were unaffected and the customers facing network interruptions where those calling or texting AT&T customers.

Digital-service tracking site DownDetector, reported service disruptions for several customers for more than 9 hours or more. This is not the first time AT&T is suffering outages, as it has over the past few days had sporadic outages according to some customers and there was a temporary 911 outage in southeastern United States areas but they were not as prolonged or nationwide.

“We have restored wireless service to all our affected customers,” announced the firm. “We sincerely apologize to them. Keeping our customers connected remains our top priority, and we are taking steps to ensure our customers do not experience this again in the future.”

On Oct. 13, 1983, AT&T launched the first commercial cellular telephone service in Chicago and ha grown to become one of the biggest telco operators and the world’s fourth-largest telecommunications company by revenue and the largest wireless carrier in the United States.