Quick take
The goal is not to block every unusual OTP request. The goal is to slow abuse, cap spend, preserve real-user conversion, and give support enough context to understand why a message was not sent.
Practical pumping controls
Layer controls so one weak signal does not punish legitimate users.
| Control | What it limits | Implementation note |
|---|---|---|
| Per-recipient limits | Repeated sends to one number | Track attempts by normalized E.164 number, user, IP, and device. |
| Country controls | Unexpected high-cost or high-risk destinations | Require explicit launch approval for each destination country. |
| Spend limits | Runaway cost exposure | Set daily and hourly caps by app, country, and use case. |
| Resend controls | User-triggered spam and accidental duplicates | Use cooldowns, idempotency keys, and one active code window. |
| Suspicious prefixes | Known high-risk ranges or anomalous routing | Review with provider and avoid hardcoded assumptions without evidence. |
| Support-visible block reasons | Mystery failures | Show safe reason categories without exposing abuse rules to attackers. |
Protect conversion while reducing abuse
A blocked OTP should not strand a legitimate user. Offer safe retry timing, alternative authentication when available, and support escalation for high-value account actions.
Do not expose fraud logic in user-facing copy. Internally, store the control that fired, the confidence level, the destination, and whether the send was skipped or allowed with monitoring.
SMS pricing by country
Compare per-message rates before choosing a provider.
Keep OTP fraud controls explainable
Notilify connects OTP sending, abuse controls, delivery evidence, and support visibility.