Why does my phone say SOS? What you should know about cell phone failures

(Story has been updated to include new information.)

Tens of thousands of cell phone users were affected by a nationwide telecommunications outage on Monday.

Verizon users were hit the hardest, according to Downdetector.com, a website that monitors outages in real time.

Phoenix was among the hardest-hit cities, with more than 5,000 Verizon customers affected. Chicago, Seattle, Omaha and Indianapolis also experienced significant impacts. The spike in outage reports began around 6:30 a.m. and peaked around 8:15 a.m. Arizona time.

Downdetector showed that more than 100,000 Verizon customers and thousands of other providers were affected nationwide. More than half of affected customers reported problems with their cell phones, while 34% reported no reception and 13% reported a total blackout.