Firebase Auth using phone number returns an internal error

Double check that the app bundle ID in Xcode matches the bundle ID in Firebase exactly. And by exactly, make sure their case matches - Xcode likes to use mixed case by default for the app name part of the bundle ID.

If you end up changing the bundle ID in Xcode, make sure to manually delete the provisioning profile for the app before generating a new one in Xcode, or it will repeatedly fail (Apple apparently ignores case on profile names).

Tested with latest Firebase iOS SDK i.e. 4.0.0 and Xcode 8.3

Firstly , remove this key FirebaseAppDelegateProxyEnabled from info.plist. This is not needed.

Now in AppDelegate.swift add following functions

import Firebase
import UserNotifications

@UIApplicationMain

class AppDelegate: UIResponder, UIApplicationDelegate , UNUserNotificationCenterDelegate{
    var window: UIWindow?
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {

        if #available(iOS 10.0, *) {
            // For iOS 10 display notification (sent via APNS)
            UNUserNotificationCenter.current().delegate = self
            let authOptions: UNAuthorizationOptions = [.alert, .badge, .sound]
            UNUserNotificationCenter.current().requestAuthorization(
                options: authOptions,
                completionHandler: {_, _ in })
        } else {
            let settings: UIUserNotificationSettings =
                UIUserNotificationSettings(types: [.alert, .badge, .sound], categories: nil)
            application.registerUserNotificationSettings(settings)
        }

        application.registerForRemoteNotifications()
        FirebaseApp.configure()
        return true
    }
func application(_ application: UIApplication, didRegisterForRemoteNotificationsWithDeviceToken deviceToken: Data) {
    // Pass device token to auth.
    let firebaseAuth = Auth.auth()

    //At development time we use .sandbox
    firebaseAuth.setAPNSToken(deviceToken, type: AuthAPNSTokenType.sandbox)

    //At time of production it will be set to .prod
}

func application(_ application: UIApplication, didReceiveRemoteNotification userInfo: [AnyHashable : Any], fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
    let firebaseAuth = Auth.auth()

    if (firebaseAuth.canHandleNotification(userInfo)){
        print(userInfo)
        return
    }
}*

Send a verification code to the user's phone:

In the class where you want to integrate Phone Authentication write :

Note : I have added +91 as its country code for India. You can add country code according to your region.

 PhoneAuthProvider.provider().verifyPhoneNumber("+919876543210") { (verificationID, error) in
       if ((error) != nil) {
             // Verification code not sent.
             print(error)
       } else {
              // Successful. User gets verification code 
              // Save verificationID in UserDefaults
             UserDefaults.standard.set(verificationID, forKey: "firebase_verification")
             UserDefaults.standard.synchronize()
             //And show the Screen to enter the Code.
       }               

Sign in the user with the verification code:

 let verificationID = UserDefaults.standard.value(forKey: "firebase_verification")
 let credential = PhoneAuthProvider.provider().credential(withVerificationID: verificationID! as! String, verificationCode: self.txtEmailID.text!)

   Auth.auth().signIn(with: credential, completion: {(_ user: User, _ error: Error?) -> Void in
         if error != nil {
            // Error
          }else {
             print("Phone number: \(user.phoneNumber)")
              var userInfo: Any? = user.providerData[0]
                    print(userInfo)
                }
         } as! AuthResultCallback)

In my case it was the apns token type that was wrong:

Auth.auth().setAPNSToken(deviceToken, type: AuthAPNSTokenType.prod)

should have been:

Auth.auth().setAPNSToken(deviceToken, type: AuthAPNSTokenType.sandbox)