Firebase Auth vs Custom OTP API: Pros & Cons

When you need phone number verification in your app, you have two fundamental approaches: use a managed authentication service like Firebase Phone Auth, or integrate a custom OTP API and build the verification flow yourself. Both approaches work, but they differ significantly in cost, flexibility, data control, and how well they serve the Indian market. This article helps you make an informed choice.

The Choice: Managed Auth vs Custom OTP

Firebase Phone Auth is part of Google's Firebase platform. It handles the entire authentication flow: generating OTPs, sending SMS, verifying codes, and managing user sessions. You get a complete auth system with minimal code.

A custom OTP API like StartMessaging handles only the OTP part: sending a code to a phone number and verifying that the user entered the correct code. You build the authentication logic, session management, and user database yourself (or integrate it with your existing auth system).

The right choice depends on where you are in your product journey and what trade-offs matter most to you.

How Firebase Phone Auth Works

Firebase Phone Auth provides:

• Client-side SDKs for web, iOS, and Android that manage the entire OTP flow
• Automatic SMS sending and code generation (you never see the OTP)
• reCAPTCHA verification to prevent abuse
• User management (Firebase creates and manages user records)
• JWT tokens for session management
• Integration with other Firebase services (Firestore, Cloud Functions, etc.)

The developer experience is straightforward: add the Firebase SDK, call signInWithPhoneNumber(), and Firebase handles everything else. The trade-off is that Firebase controls the entire flow.

How a Custom OTP API Works

With a custom OTP API like StartMessaging, your backend:

1. Receives a phone number from your frontend
2. Calls the OTP API to send a code to that number
3. Receives the code back from the user
4. Calls the OTP API to verify the code
5. Creates or authenticates the user in your own database
6. Issues your own session token (JWT, cookie, etc.)

You write more code, but you control every aspect of the flow: the UI, the timing, the retry logic, the user model, and the session management.

Pricing Comparison

Firebase Phone Auth uses a verification-based pricing model with a free tier:

Firebase's free tier (50 verifications/day, approximately 1,500/month) is attractive for very early-stage projects. However, once you exceed the free tier, costs escalate quickly. Firebase charges $0.01 for the first 10,000 verifications in a region, then $0.06 per verification after that. For India at scale, this becomes significantly more expensive than a dedicated OTP API.

StartMessaging has no free tier, but at Rs 0.25 per OTP, even 1,500 verifications per month costs only Rs 375. The predictable, flat pricing makes budgeting straightforward. See the full details on our pricing page.

Vendor Lock-in and Portability

This is one of the most important considerations and is often overlooked when choosing Firebase.

Firebase lock-in: When you use Firebase Phone Auth, your user identities live in Firebase. Your app depends on Firebase SDKs. Your auth tokens are Firebase JWTs. If you decide to migrate away from Firebase (to your own backend, to Supabase, to AWS Cognito, etc.), you need to:

• Export all user data from Firebase (limited by Firebase export capabilities)
• Rebuild your auth flow with a new provider
• Migrate user sessions without forcing everyone to re-authenticate
• Update all client-side SDK calls
• Handle the transition period where users might be on old or new auth

Custom OTP API portability: When you use a custom OTP API, your user data lives in your own database. Your auth flow is in your own code. If you want to switch OTP providers (from StartMessaging to MSG91 or vice versa), you change a few API calls in your backend. Your user model, sessions, and frontend remain untouched. The OTP provider is a pluggable service, not a foundation.

Customization and UX Control

Firebase provides a pre-built UI flow. While you can customize the appearance somewhat, the fundamental flow is controlled by Firebase:

• reCAPTCHA is mandatory (visible or invisible) — you cannot remove it
• OTP length, format, and expiry are controlled by Firebase
• The SMS message template is fixed (“Your verification code is...”)
• Retry and rate limiting logic is Firebase's, not yours
• You cannot customise what happens between send and verify

Custom OTP API gives you full control:

• Design your own verification UI exactly how you want it
• Control retry logic, cooldown periods, and attempt limits
• Implement custom rate limiting per user, IP, or device
• Add your own abuse prevention (no mandatory reCAPTCHA)
• Build custom flows like OTP + password, OTP + biometric, or OTP as second factor
• Use idempotency keys to prevent duplicate sends (StartMessaging supports this natively)

India-Specific Delivery Performance

Firebase Phone Auth uses Google's own SMS infrastructure, which routes messages through international gateways. For India, this means:

• Messages may be routed internationally, adding latency
• DLT compliance is handled by Google but delivery routing may not be optimised for Indian carriers
• Delivery rates on some carriers (particularly BSNL and Vi) can be inconsistent
• Users occasionally report delays of 30-60 seconds for OTP delivery

StartMessaging uses domestic Indian SMS providers with carrier-specific optimisations:

• Messages routed through domestic gateways for lower latency
• Automatic provider fallback if the primary route fails
• 97-99% delivery rates across Jio, Airtel, Vi, and BSNL
• Typical delivery time of 3-8 seconds

For a detailed breakdown by carrier, see our OTP delivery rates in India guide.

Data Residency and Privacy

Firebase stores user data on Google Cloud infrastructure. While Google has data centres in India (Mumbai region), Firebase does not guarantee that all user authentication data is stored exclusively in India. For companies with strict data residency requirements (fintech, healthcare, government projects), this can be a compliance concern.

With a custom OTP API approach, your user data lives in your own database, hosted wherever you choose. The OTP provider (StartMessaging) only processes the phone number and OTP code transiently — it does not maintain a persistent user identity store. This separation of concerns makes it easier to comply with data localisation requirements.

Code Comparison

Firebase Phone Auth (React)

import { getAuth, signInWithPhoneNumber, RecaptchaVerifier } from 'firebase/auth';

// Setup reCAPTCHA (required)
const auth = getAuth();
const recaptcha = new RecaptchaVerifier(auth, 'recaptcha-container', {
  size: 'invisible',
});

// Send OTP
const confirmationResult = await signInWithPhoneNumber(
  auth,
  '+919876543210',
  recaptcha
);

// Verify OTP (user enters code)
const result = await confirmationResult.confirm('482916');
const user = result.user; // Firebase user object
const token = await user.getIdToken(); // Firebase JWT

Custom OTP with StartMessaging (Node.js backend)

// Backend: Send OTP
app.post('/auth/send-otp', async (req, res) => {
  const { phoneNumber } = req.body;

  const response = await fetch('https://api.startmessaging.com/otp/send', {
    method: 'POST',
    headers: {
      'Authorization': 'Bearer sm_live_your_api_key',
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({ phoneNumber }),
  });

  const { data } = await response.json();
  res.json({ requestId: data.requestId });
});

// Backend: Verify OTP
app.post('/auth/verify-otp', async (req, res) => {
  const { requestId, otp } = req.body;

  const response = await fetch('https://api.startmessaging.com/otp/verify', {
    method: 'POST',
    headers: {
      'Authorization': 'Bearer sm_live_your_api_key',
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({ requestId, otp }),
  });

  const { data } = await response.json();

  if (data.verified) {
    // Create or find user in YOUR database
    const user = await findOrCreateUser(phoneNumber);
    // Issue YOUR JWT token
    const token = jwt.sign({ userId: user.id }, process.env.JWT_SECRET);
    res.json({ token });
  }
});

The Firebase approach is fewer lines of code and runs on the client side. The custom OTP approach requires a backend but gives you complete ownership of the user model and auth tokens.

Comparison Table

When to Use Which

Use Firebase Phone Auth when:

• You are building an MVP or prototype and need auth up and running in hours
• You are already invested in the Firebase ecosystem (Firestore, Functions, Hosting)
• Your volume is under 50 verifications per day (free tier)
• You need social login (Google, Apple, Facebook) alongside phone auth
• You do not have a backend or do not want to build auth logic

Use a custom OTP API (StartMessaging) when:

• You are building for the Indian market and need the best delivery rates and speed
• You want to avoid vendor lock-in and own your user data
• You are scaling beyond the free tier and want predictable, low costs
• You need full control over the verification UX and flow
• You have data residency requirements that need Indian hosting
• You already have a backend and want OTP as a service, not a full auth system
• You want INR billing without forex overhead

Hybrid approach: Some teams use Firebase for social login and account management, but use a custom OTP API for phone verification. This gives you the best of both worlds: Firebase for Google/Apple sign-in, and StartMessaging for reliable, affordable OTP in India.

Ready to try the custom OTP approach? Check out the StartMessaging API documentation or see how we compare to other providers in our best OTP API for India guide.