How to fix OTP not receiving issue in SIM registration web app (JavaScript + API)?

I am building a SIM registration web application where users enter their mobile number and receive an OTP for verification. However, I am facing an issue where the OTP is sometimes not received, even though the API response shows success.

Setup:

Frontend: JavaScript (Vanilla JS)

Backend: Node.js (Express)

SMS API: Third-party gateway (REST API based)

Problem:

API returns status 200 (success)

OTP is generated correctly on the server

But users do not receive OTP on their phone in some cases

Code Snippet:

async function sendOTP(number) { const response = await fetch('/api/send-otp', { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify({ phone: number }) }); const data = await response.json(); console.log(data); }

What I have tried:

Verified API key and SMS gateway balance

Checked number format (with country code)

Tested with multiple numbers

Logged server-side OTP generation

Questions:

What could cause OTP delivery failure despite successful API response?

Are there common rate limits or filtering issues in SMS gateways?

How can I implement retry or fallback logic for better reliability?

Any suggestions or debugging strategies would be helpful.

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