Did you connect your handset to the phone port 1 of the NBN router? If not, please connect it to its designated port for the service to work.
Here's an article that will guide you with the NBN FTTC setup.
The back to base alarm or any other old dial up technology devices will not work with the NBN technology.
You may refer to:
Important things you need to know > Frequently Asked Questions > TPG Voice Service (https://www.tpg.com.au/nbn)
7. Once TPG NBN Voice Service is installed, are there any services that will not work on my TPG Voice Service?
TPG Voice Service does not support old dial up technology devices with built in modems such as EFTPOS, Back to Base Alarms and Foxtel Program purchase. These devices have been replaced by Internet Protocol (IP) enabled devices and many TPG customers are able to use EFTPOS, Back to Base alarms and Foxtel Program purchase through the internet provided that their devices are IP enabled. For further details on IP enabled devices contact your device supplier. TPG Voice Service also does not support analogue modems, Priority Assistance Scheme, calls to 19/1900, 0500 numbers and Universal International Freephone Numbers (UIFN).