(Bloomberg) — Verizon Communications Inc. (VZ) said it would issue $20 credits to customers affected by a widespread service outage on Wednesday, which it attributed to a “software issue.”

“Yesterday, we did not meet the standard of excellence our customers expect and that we expect of ourselves,” Verizon said in a statement Thursday morning. The company is offering $20 account credits, which should cover “multiple days of service.”

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People began noticing service disruptions shortly before noon in New York, according to Downdetector. Complaints peaked at roughly 177,339 about an hour later, according to the site. The cities that had the largest number of complaints included New York, Houston, Atlanta, Dallas and Miami.

AT&T Inc. (T) and T-Mobile US Inc. (TMUS) customers were reporting a much smaller number of network issues, according to Downdetector. When one large carrier has issues, a residual effect is that calls to that company’s customers can’t be completed.

The Federal Communications Commission said on X that it was aware of the outages and was monitoring the situation. Separately, FCC member Anna Gomez said she is “paying close attention” to the reports and that she will ask the agency’s consumer and public safety bureaus to “investigate the source of this service disruption.”

Last August, Verizon suffered another significant network outage that affected thousands of customers across the US and disrupted service for several hours. The company said at the time the problem was due to a software issue.

Such disruptions are generally caused by external factors, not carriers themselves, according to David Witkowski, a senior member of the Institute of Electrical and Electronic Engineers, an organization that helps to set global technological standards.

Cyberattacks can be one external factor leading to a disruption, he said in an emailed statement, but issues can also stem from problems within third-party vendors that “provide contracted compute resources.” Other external factors can include “software updates and patches that work on testbed systems, but fail in deployment due to unforeseen issues with legacy hardware,” he said.

Verizon said on Wednesday that it had no indication to suggest it could be related to a cyberattack.