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Your NBN FTTN service is only provisioned to have 1 working phone port which is the phone port 1.
Should you need to connect multiple handsets, you can try using wireless handsets where in the base unit is is connected to the phone port 1. To add an additional handset to your cordless phone system, you first need to consult its documentation to determine the model of a compatible add-on unit. Once you have the add-on unit, register it with your cordless phone system to begin using it.
Another option is you may consult a licensed cabler in order to connect the home's telephone cabling to the modem for multiple access.
Let us know should you require additional assistance.