---
title: "Designing Voice Call Fallback for Unreachable SMS Verification Codes"
---

Designing Voice Call Fallback for Unreachable SMS Verification Codes: SMSRiver

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OTP & 2FA Engineering

Published 2026-09-08

•

7 min read

# Designing Voice Call Fallback for Unreachable SMS Verification Codes

Implement automated Text-to-Speech (TTS) voice call fallback when SMS codes fail to arrive. Learn call cadence, DTMF security, and carrier accessibility guidelines.

Key Takeaway & Executive Summary

Voice call OTP fallback recovers up to 18% of users with landlines, aggressive SMS spam filters, or impaired cellular text messaging reception.

When SMS delivery fails due to carrier spam filtering or landline phone numbers, voice call verification offers a reliable secondary channel. By dispatching an automated outbound call that speaks the passcode using Text-to-Speech, applications ensure high conversion across edge demographics.

## Table of Contents

- [Voice OTP Architecture and Pacing](#section-1)

- [Frequently Asked Questions](#frequently-asked-questions)

## 1 Voice OTP Architecture and Pacing

Voice calls require careful audio design:

- Initial Pause: Include a 1.5-second initial silence before speaking to prevent words being swallowed during handset pickup.
- Clear Cadence: Speak numbers individually with 0.5-second pauses ('Your code is: 4... 8... 2... 9... 1... 0').
- Automatic Repeat: Repeat the full sequence twice.
- DTMF Keypress Gate: Optionally require the user to press a key (e.g. 'Press 1 to hear your passcode') to defeat automated voicemail recordings from capturing passcodes.

## Frequently Asked Questions • OTP & 2FA Engineering

Are voice OTP calls more expensive than SMS?

Voice calls typically cost $0.015 to $0.03 per minute, roughly 2x to 4x the cost of a domestic SMS segment. However, because voice is only used as a fallback, it preserves high conversion.

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