Firebase phone authentication iOS Token Mismatch error

I’m building an app with Firebase phone authentication using this tutorial https://medium.com/@gbrigens/ionic-3-phone-authentication-with-firebase-dbed967e95ef. It works fine on an Android device, but on an iPhone, when I call

FirebasePlugin.verifyPhoneNumber()

I get this Token Mismatch error…

ERROR: {“description”:“Error Domain=FIRAuthErrorDomain Code=17048 “Token mismatch” UserInfo={NSLocalizedDescription=Token mismatch, error_name=INVALID_APP_CREDENTIAL}”,“code”:17048}

I have no clue how to resolve this. Any help would be greatly appreciated.

Thanks.