SMS provider carrier-specific degradation

At 19:20 new-user signups and 2FA logins start failing at the SMS step. Dashboards: your SMS provider's API for sending OTP codes returns elevated 500s and high latency for one route (carrier/region specific); login-completion rate for users requiring SMS 2FA drops sharply; users with app-based authenticator codes are unaffected. The provider's status page notes degraded delivery to certain carriers. Recent context: none on your side. How do you triage and mitigate?