"We basically let our customers down and I apologize to each and every one of them," said C. Michael Armstrong, AT&T's chairman and chief executive.
The telecommunications giant experienced what it termed "service interruptions" nationwide starting at 12 p.m. PT yesterday, apparently affecting thousands of business customers using the company's Interspan frame relay service, according to AT&T spokespeople.
The company declined to disclose the exact number of customers affected by the outage due to competitive concerns. Customers will not be charged for use of the network until a root cause of the problem is found.
CNET Radio talks to AT&T's Adele Ambrose
"Cisco shares AT&T's commitment to providing the highest quality communications services. We view this interruption as unacceptable, and apologize to our joint customers who have been affected," Don Listwin, senior vice president of Cisco's Service Provider line of business, said in a statement. "Ensuring the highest level of network availability for our customers is always our top priority."
As of 11:45 a.m. PT today, 96 percent of the network had been restored, according to AT&T.
Frame relay technology offers a high-performance form of packet switching that is generally regarded as more reliable for the needs of organizations sending large "bursts" of data. The technology is generally deployed in wide area networks across far-flung geographic regions.
Reuters contributed to this report.