The carrier level issues affecting T-Mobile are now confirmed to be fully resolved.
Update - Duo has identified this as a carrier level issue affecting T-Mobile. They are working with their partners and the carrier to resolve this as quickly as possible.
From Duo vendor - status.duo.com:
We have identified issue with Phone Call and SMS deliveries to T-Mobile phone numbers on all of our deployments and are working to correct the issue as soon as possible.
All other authentication factors work as per usual (Duo Push, U2F, hardware token, etc.)
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