As of 9:35 a.m., 73,000 customers had reported an issue, according to Downdetector.
The company said it was "working urgently" to restore service and encourages the use of Wi-Fi calling.
Customers began reporting issues in the early-morning hours, but the issue lasted well into the afternoon for thousands, Downdetector shoes.
The issue had resolved for most customers by 2 p.m.
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