Telephony service provider is reporting the issue as solved.
Our third party telephony provider is reporting delivery issues for SMS from 10DLC numbers towards AT&T and Verizon networks in USA.
https://vonageapi.statuspage.io/incidents/y3x949vzh4s5
One alternative is to use email OTP: https://www.logintc.com/docs/guides/email-otp/
Another to SMS, users can receive a phone call with an OTP:
https://www.logintc.com/docs/guides/phones/#phone-call-otp-authentication
For the LoginTC RADIUS Connector this requires an upgrade to 4.1.0: