asynchronous guided access app still allowing phone call alerts

I have an app that uses asynchronous guided access mode to lock down phone into application. However notifications of phone calls still come through. We can enable Privacy Mode which will mute phone calls while Guided Access is disabled. But when it is locked the alerts/sounds still come through? Any suggestions as to what may be happening. Guided Access is enabled and it has been tested with Privacy On and Off as well as notifications for the Phone App to be disabled. All lead to alerts being presented when in house app is in guided access mode.