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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).
The reports for AT&T numbered around 32,000 - with hotspots in the southern and eastern US, said Downdetector.
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 the operators for comment.
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.