Firebase SMS Verification on iOS - 'Token Mismatch'

Steps I took to solve the problem:

1. Ensure I had both a valid development and production APNS certificate uploaded to the Firebase Dashboard, under 'Project Settings' > 'Cloud Messaging'

2. In Xcode, in the <my app>.entitlements file, make sure the APS Environment value is set to either 'development' or 'production', depending on your testing situation.

3. Finally (this is what I was missing), check inside your AppDelegate.swift and inside the method for didRegisterForRemoteNotificationsWithDeviceToken, change the value from .sandbox to .prod, or to .unknown to let the app bundle determine which token type to use, based on your provisioning profile.

Step 1: import Firebase

2. Add the following to your AppDelegate:-

override func application(_ application: UIApplication, didRegisterForRemoteNotificationsWithDeviceToken deviceToken: Data) {
    let firebaseAuth = Auth.auth()
    firebaseAuth.setAPNSToken(deviceToken, type: AuthAPNSTokenType.unknown)

}

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

At least this worked for me.