So your need is really involving two areas. Auth.net does have a preset set of card numbers (similar to the one you’re using) that are designed to returns specific results for testing. However, IS has a front end that preempts that process and it may use the LUND algorithm or other validation to first check the number so it may interfere with the backend auth.net process.
@martinc, do you know or can you find out if IS has pre merchant account validation that would prevent the use of the auth.net test numbers?