Interruptions to US mobile services have been reported across the United States on Thursday morning.
Downdetector.com,
which tracks outages, showed thousands of reports made by users of
multiple networks after 04:00 EST (09:00 GMT).
Tens of thousands of reports have been sent for AT&T - with hotspots in the south and east, Downdetector says.
Users say their phones are displaying the SOS message, leaving them unable to make calls or access services.
The cause is unclear. The BBC has approached a number of operators for comment.
AT&T is responding to customer complaints online but has not yet widespread acknowledged network problems.
Verizon - one of the networks that has received reports on Downdetector - told the BBC its network was operating normally.
It suggested customers were logging issues after making unsuccessful attempts to contact users of another provider.
T-Mobile
said its network is also working as normal and customers reporting
issues may be having trouble making outgoing calls to another network.
A few thousand customers with Cricket Wireless, UScellular and Consumer Cellular are also reporting issues.
Users are having issues with calls, texts and internet access, with most reports indicating no service or no signal.
Downdetector's
heatmap shows major population centres including Los Angeles, Chicago,
Houston and Atlanta are among those facing the highest number of
outages.
911
emergency service is also down in some areas, with officials advising
the use of a landline, social media or a cell phone with a different
carrier in the event of an emergency.
