func auth(phoneNumber: String,
          onSuccess: @escaping () -> (),
          onError: @escaping (Error) -> ()) {
    
    PhoneAuthProvider.provider()
      .verifyPhoneNumber(phoneNumber, uiDelegate: nil) { [weak self] verificationID, error in
          if let error = error {
              onError(error)
          } else if let verificationID = verificationID {
              self?.userDefaults.firebaseAuthVerificationID = verificationID
              onSuccess()
          }
      }
}