The Duo issue regarding Phone Call Delivery is fully resolved and the service is back to normal.
Update: 10:13 a.m. -- The Duo issue regarding Phone Call Delivery is fully resolved. You may visit Duo's status page for more details on this incident.
9:49 a.m. -- A fix has been implemented by Duo's telephony providers and they are currently monitoring the results. If users are still having issues, please contact the IT Support Center at (310) 267-HELP (4357) or firstname.lastname@example.org.
9:29 a.m. -- Duo has observed issues with key presses not being registered for phone call authentications, as well as calls being terminated due to no audio being received. At this time, the issue appears to only affect phone call authentications to North American phone numbers.
Duo has reached out to their telephony providers to further investigate this behavior and are working with them to resolve the issue.