(Reuters) – Tens of thousands of Verizon users reported outages in the carrier’s services across the U.S. on 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 around 9:30 a.m. ET 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 is aware of an issue affecting its services. “Our engineers are committed and we are working quickly to identify and resolve the issue.”

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

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

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

“Downdetector likely reflects the challenges our customers face when trying to connect to users on a different network,” AT&T said in a post on X.

In February, industry rival AT&T experienced nationwide cell phone outages that lasted more than 12 hours and affected more than 70,000 customers.

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

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

(Reporting by Harshita Mary Varghese and Aditya Soni in Bengaluru; Editing by Anil D’Silva, Tasim Zahid and Devika Syamnath)