Firebase phone auth if sms is not received

I'm using Firebase phone auth for login in my application in production.. but many times I encounter issues with local carriers which makes SMS OTP not received by users on a wide range (both new and already registered users).

I'm looking for an idea to solve such issue using phone login.. Eg. sending OTP code over Whatsapp using my custom API..

Any idea how to send firebase auth OTP code using Whatsapp?

In other words; How could I know the OTP code that I should send to the user from my backend?

I've looked for a solution using firebase auth admin API But couldn't find an answer

Ok, let's try to help you:

As you already have user's phone so:

• in your database get user by its phonenumber
• manage to create the 6 random digits authentication code and store this data in your database. Maybe something like this:

- authentication
  - user uid
    - code

EDIT: Instead of storing the code you can create a JWT token with your code as payload and with a expiration (for security purposes) of 30 minutes for example. And then when your user sends the code to your api you can check if this code is expired when decoding your jwt token. Also you don’t need to worry about deleting the code because after 30min the token is expired so you have total control.

Then send this code to your user by email, by whatsapp or any other method you prefer.

Now to validate this code you can create a query to your database (I'm suposing you're using firestore):

// again get user data by it's phonenumber here
const {uid} = ....

const userAuthDoc = await admin.firestore().doc(`authentication/${ uid }`).get()

// now here you can check if the code provided 
// by the user is the same as you have stored

And now you know if user is authenticated or not based on provided code... then you can create a custom token and send it back to your user as response from code validation api endpoint.

Now on mobile code you can use the signInWithCustomToken authentication method.

OBS: this link leads to web docs but native ios/android and also RN docs have this method

I know my answer is not so complete but I just want to give you some idea of how to solve this problem.