How to authenticate Mobile Number using Firebase in Android Studio using Kotlin.

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FirebaseOtpLogin > app > right-click > paste.

 implementation 'com.google.firebase:firebase-auth:21.0.1'

apply plugin: 'com.google.gms.google-services'

 classpath 'com.google.gms:google-services:4.3.10'

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="20dp"
    android:gravity="center"
    tools:context=".MainActivity">

    <EditText
        android:id="@+id/mobileNumber"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:ems="10"
        android:hint="Enter your Mobile Number"
        android:inputType="phone" />

    <Button
        android:id="@+id/button"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginStart="100dp"
        android:layout_marginLeft="100dp"
        android:layout_marginTop="20dp"
        android:layout_marginEnd="100dp"
        android:layout_marginRight="100dp"
        android:text="Send OTP"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/mobileNumber" />
</LinearLayout>

class MainActivity : AppCompatActivity() {
    var number: String = ""

    // create instance of firebase auth
    lateinit var auth: FirebaseAuth

    // we will use this to match the sent otp from firebase
    lateinit var storedVerificationId: String
    lateinit var resendToken: PhoneAuthProvider.ForceResendingToken
    private lateinit var callbacks: PhoneAuthProvider.OnVerificationStateChangedCallbacks

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        val sendOtp = findViewById(R.id.button)

        auth = FirebaseAuth.getInstance()

        // start verification on click of the button
        sendOtp.setOnClickListener {
            login()
        }

        // Callback function for Phone Auth
        callbacks = object : PhoneAuthProvider.OnVerificationStateChangedCallbacks() {

            // This method is called when the verification is completed
            override fun onVerificationCompleted(credential: PhoneAuthCredential) {
                startActivity(Intent(applicationContext, MainActivity::class.java))
                finish()
                Log.d("Msg", "onVerificationCompleted Success")
            }

            // Called when verification is failed add log statement to see the exception
            override fun onVerificationFailed(e: FirebaseException) {
                Log.d("MSg", "onVerificationFailed  $e")
            }

            // On code is sent by the firebase this method is called
            // in here we start a new activity where user can enter the OTP
            override fun onCodeSent(
                verificationId: String,
                token: PhoneAuthProvider.ForceResendingToken
            ) {
                Log.d("Msg", "onCodeSent: $verificationId")
                storedVerificationId = verificationId
                resendToken = token

                // Start a new activity using intent
                // also send the storedVerificationId using intent
                // we will use this id to send the otp back to firebase
                val intent = Intent(applicationContext, OTPActivity::class.java)
                intent.putExtra("storedVerificationId", storedVerificationId)
                startActivity(intent)
                finish()
            }
        }
    }

    private fun login() {
        number = findViewById(R.id.mobileNumber).text.trim().toString()

        // get the phone number from edit text and append the country cde with it
        if (number.isNotEmpty()) {
            number = "+91$number"
            sendVerificationCode(number)
        } else {
            Toast.makeText(this, "Enter mobile number", Toast.LENGTH_SHORT).show()
        }
    }

    // this method sends the verification code
    // and starts the callback of verification
    // which is implemented above in onCreate
    private fun sendVerificationCode(number: String) {
        val options = PhoneAuthOptions.newBuilder(auth)
            .setPhoneNumber(number) // Phone number to verify
            .setTimeout(60L, TimeUnit.SECONDS) // Timeout and unit
            .setActivity(this) // Activity (for callback binding)
            .setCallbacks(callbacks) // OnVerificationStateChangedCallbacks
            .build()
        PhoneAuthProvider.verifyPhoneNumber(options)
        Log.d("Msg", "Auth started")
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:gravity="center"
    android:padding="20dp"
    tools:context=".OTPActivity">

    <EditText
        android:id="@+id/otpTextField"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:ems="10"
        android:hint="Enter OTP"
        android:inputType="number" />

    <Button
        android:id="@+id/verifyButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Verify" />
</LinearLayout>

class OTPActivity : AppCompatActivity() {
    // get reference of the firebase auth
    lateinit var auth: FirebaseAuth

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_otpactivity)

        auth=FirebaseAuth.getInstance()

        // get storedVerificationId from the intent
        val storedVerificationId= intent.getStringExtra("storedVerificationId")

        // fill otp and call the on click on button
        findViewById(R.id.verifyButton).setOnClickListener {
            val otp = findViewById(R.id.otpTextField).text.trim().toString()
            if(otp.isNotEmpty()){
                val credential : PhoneAuthCredential = PhoneAuthProvider.getCredential(
                    storedVerificationId.toString(), otp)
                signInWithPhoneAuthCredential(credential)
            }else{
                Toast.makeText(this,"Enter OTP", Toast.LENGTH_SHORT).show()
            }
        }
    }
    // verifies if the code matches sent by firebase
    // if success start the new activity in our case it is main Activity
    private fun signInWithPhoneAuthCredential(credential: PhoneAuthCredential) {
        auth.signInWithCredential(credential)
            .addOnCompleteListener(this) { task ->
                if (task.isSuccessful) {
                    val intent = Intent(this , HomeActivity::class.java)
                    startActivity(intent)
                    finish()
                } else {
                    // Sign in failed, display a message and update the UI
                    if (task.exception is FirebaseAuthInvalidCredentialsException) {
                        // The verification code entered was invalid
                        Toast.makeText(this,"Invalid OTP", Toast.LENGTH_SHORT).show()
                    }
                }
            }
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:gravity="center"
    android:background="#DCC9C9"
    tools:context=".HomeActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello Guys! Welcome"
        android:textColor="@color/purple_700"
        android:textSize="29sp"></TextView>
</LinearLayout>

https://console.cloud.google.com/apis/library/androidcheck.googleapis.com?pli=1

Sometimes Android Studio throws this issue:
If you got any error of this type: 
     Failed to get reCAPTCHA token with error [There was an error while trying to get your package certificate hash.]- calling backend without app verification.