RCT_EXPORT_METHOD(verifyPhoneNumber:(NSString *)phone callback:(RCTResponseSenderBlock)callback)
{
  [[FIRPhoneAuthProvider provider] verifyPhoneNumber:phone
                                          completion:^(NSString * _Nullable verificationID, NSError * _Nullable error) {
    if (error) {
      callback(@[error, [NSNull null]]);
      return;
    }

    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
    [defaults setObject:verificationID forKey:@"authVerificationID"];

    callback(@[[NSNull null], verificationID]);
  }];
}

RCT_EXPORT_METHOD(getCredential:(NSString *)code callback:(RCTResponseSenderBlock)callback)
{
  NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
  NSString *verificationID = [defaults stringForKey:@"authVerificationID"];

  FIRAuthCredential *credential = [[FIRPhoneAuthProvider provider]
    credentialWithVerificationID:verificationID
                verificationCode:code];

  callback(@[[NSNull null], credential]);
}

MyNativeModule.getCredential("12345", (error, credential) => {
    ...
    firebase.auth().signInWithCredential(credential)
});