SMS does not arrive if message include OTP autocomplete

Question

I create a custom auth for a Cognito User Pool, And send users an SMS via SNS formatted as

```
Your authentication code is 123456

@domain.com #123456
```

If I remove the last line, the message arrives, but if not, it never arrives, but SNS never reports an error, and the logs says it was accepted by the phone number, or sometimes by the carrier.

How can I send the original message with the autocomplete line?

Kidd Ip · Answer

Below was what I hit in Hong Kong region as the past.

This is related to a Mobile carrier-level content filtering issue:

•	Mobile carriers and\or SMS aggregators apply content filtering rules to block messages that look like spam, phishing, or automated login attempts.
•	The line @domain.com #123456 matches patterns used by SMS OTP auto-complete formats (Android/iOS use this convention to auto-fill codes).
•	Some carriers treat this syntax as suspicious or “application-to-person” traffic that requires special registration. As a result, they silently drop the message even though SNS reports it as “delivered to carrier.”
•	That’s why removing the last line makes the SMS arrive, the message no longer triggers the filter.

SMS does not arrive if message include OTP autocomplete

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