This is not a question but a solution for the record. 10 calls the Telstra Business support could not solve this issue but the solution is here --
[Problem] Phone service was not working and the phone connection indicator on the modem was blinking orange. No phone service, just an engaged signal.
[Solution] Use web browser to configure modem (192.168.15.1) and login to admin. Click Advanced settings, select Setup tab and then Voice-call-setup. Choose Manual (but don't press Apply) and see if there are any settings. If the setting are missing, don't enter anything, select Auto-Configure again and then select the Basic tab again. Wait for a few minutes to check if the Voice LED has turned blue.
[Check] Return to Voice-call-setup and select Manual again. If you still don't have any settings it is likely you need to return to factory default setting using the reset button.
Hello