Voice OTP is most effective when it is part of a defined delivery policy rather than an isolated channel.
Choose the trigger
A fallback can start after a delivery timeout, a provider error or a user request. The trigger should reflect the urgency and risk of the authentication flow.
Keep the call clear
The prompt should identify the service, read the code at a usable pace and support the languages required by the audience. Avoid adding unnecessary information.
Measure the complete journey
Track SMS attempts, call attempts and the final verification result together. Channel-level reports alone do not show whether the user successfully completed authentication.