SMS Receipt Webhook doesnt invoke

Im setting up an internal webhook-only application to act upon received SMS messages.  I verified my webhook endpoint is able to receive POST messages but it seems Zoom is not invoking it. I have the webhook subscribed to both SMS receive and send events and I have tried to use a Power-Pack enabled user and call queue for this... nothing seems to work. All the SMS functions work but the webhook is never triggered and no webhook logs are created. What could I do to further troubleshoot?