Verizon late Monday afternoon reported progress in resolving a “network issue” that caused widespread outages in parts of the U.S., including the Chicago area.

In a post on

“We know how much people rely on Verizon and apologize for any inconvenience this may have caused some of our customers today,” the company's statement continued. “We continue to work around the clock to fully resolve this issue.”

The problem appeared to have knocked out cell service for tens of thousands of Verizon users. Data from outage tracker DownDetector shows that just after 10 a.m. CT, the number of reports topped 100,000 — and while that number dropped significantly, nearly 48,000 were still experiencing problems as of 3 p.m. CT. In the Chicago area, more than 4,000 customers reported no service at the height of the outage – around 10:30 a.m. At 2:30 p.m. the number of reports dropped to around 1,200.

An outage map from Downdetector showed the “most commonly reported” outage locations are Chicago, Indianapolis, Minneapolis, Phoenix, Omaha, Cincinnati, Denver, Seattle and Columbus.

The Federal Communications Commission, the nationwide agency that regulates interstate and international communications, acknowledged the outages, saying in part: “…We are working to determine the cause and extent of these service interruptions.”

Some Verizon iPhone customers began seeing SOS messages appear in the status bar of their cell phones. The message indicates that the device is having trouble connecting to the carrier's network, but can make emergency calls through other carriers.

In the event of cellular network outages, an alternative is to connect to a nearby Wi-Fi network. Wi-Fi calling is also a built-in feature on most Android devices and iPhones and can be enabled in the phone settings.