Tens of thousands of Verizon users reported outages for the wireless carrier's services across the U.S. Monday, 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 and there were 69,753 reports as of</p> <p>12:17 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.

Some Verizon users said on 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.

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.