When SMS Doesn't Arrive: Voice Call Authentication as an Alternative
SMS is widely used for identity verification.
However, situations where “SMS doesn’t arrive” or “SMS can’t be found” do occur:
- International SMS blocking settings
- Carrier network delays
- Messages being routed into apps like Plus Message
- Users who rely on landline phones rather than mobile devices
In these cases, voice call + text-to-speech authentication is a practical and reliable alternative.
How It Works
Voice call authentication places an automated call to the user’s phone number and
reads out the verification code using TTS (text-to-speech):
“This is your verification service. Your code is 1 2 3 4.”
- The user simply answers the call
- Works on both mobile phones and landlines
- Stable in areas where SMS delivery may be unreliable
- Easy to integrate alongside SMS verification
When Voice Authentication Is Especially Useful
① Environments where SMS is unstable
International SMS restrictions, Plus Message routing, or other settings
may cause SMS to be delayed or undelivered.
② Users with only a landline phone
Reception desks, elderly users, or certain business environments
may not use mobile phones at all—but voice calls always work.
③ Users who have difficulty checking codes visually
Since the code is read aloud, voice authentication improves accessibility.
Cost & Implementation
With Xoxzo’s Voice API, voice call authentication costs:
- About 10 JPY per call (japanese domestic, see pricing page)
- The similar price range as SMS verification
- Easy to add to existing authentication flows
Documentation: https://docs.xoxzo.com/en/voice/
Comparison: SMS vs Voice Call Authentication
| Item | SMS Authentication | Voice Call Authentication |
|---|---|---|
| Delivery method | SMS app | Phone call (audio) |
| Works with landline | ✕ | ◯ |
| Affected by international SMS blocking | Yes | Less likely |
| Affected by carrier delays | Yes | Less likely |
| Accessibility | Requires reading text | Code is spoken aloud |
| Cost | ~10 JPY | see pricing page |
Implementation Ideas
- Show SMS and voice call authentication options on the login screen
- Provide voice call as a fallback when SMS verification fails
- Use voice-first authentication for users with landlines
This reduces verification failure and ultimately
helps prevent user drop-off during onboarding.
Summary
- SMS delivery can be unstable depending on device and carrier
- Voice call authentication is a highly reliable alternative
- Works with landlines and improves accessibility
- Simple to implement with Xoxzo’s Voice API
Related Links
Aiko Yokoyama
Customer Success and Operations
Joined January 2014. Experienced as a clerk in Foreign Trading company, started and maintained an online supplement store. Lived in overseas for 15 years. Looking forward to communicating our customers with the broad view based on those experience.
