Tens of thousands of Verizon users reported outages for the wireless carrier’s services across the U.S. today, with Chicago and Indianapolis among the hardest-hit cities and some iPhone users stuck in “SOS” mode.

According to tracking website Downdetector.com, the outage began at around 9:30 a.m. ET (3:30 a.m. Hawaii time) and there were 66,761 reports as of 12:28 p.m. ET, with regions including Minneapolis, Phoenix, Omaha and Denver among the most reported locations.

Verizon Communications said it was aware of an issue impacting its services. “Our engineers are engaged and we are working quickly to identify and solve the issue.”

Some Verizon users said on social media platform X their phones were stuck in “SOS” mode.

“SOS” appears in the status bars of iPhones if the device is not connected to a cellular network but can still make emergency calls through other carriers, according to Apple’s website.

The outage tracking website also showed 1,111 incident reports by AT&T users as of 12:25 p.m. ET (6:25 a.m. Hawaii time), but the carrier said it was not experiencing a nationwide outage and the network was operating normally.

“Downdetector is likely reflecting challenges our customers are having attempting to connect to users on another network,” AT&T said in a post on X.

In February, sector rival AT&T faced nationwide wireless outages that lasted over 12 hours and impacted more than 70,000 customers.

The Federal Communications Commission is investigating the AT&T outage which blocked more than 92 million voice calls and prevented more than 25,000 attempts to reach 911, the agency said.

News of the Verizon outage comes hours after the company announced a deal to give infrastructure firm Vertical Bridge rights to lease, operate and manage 6,339 mobile towers across the U.S. for $3.3 billion.