Welcome to the Community!
I pulled up your account using your community details and it seems like you have already raised the same concern with our Helpdesk Team.
This issue has been raised to our Engineering Team already since it will require assistance from NBNCo. I performed some tests and I can see that not only is your phone not working properly, but your internet connection is also intermittent. The diagnostic tests show a problem from the line going to your NBN box itself. An NBN technician will need to be dispatched to rectify this. Please wait for an update from our Engineering team pertaining to the schedule. You will be notified via SMS or phone call about it.
Should you have other questions or need clarifications, feel free to leave a message on this thread.