Before Australia switched to the digital network, your fax number corresponded with a fax landline. As these lines are part of the old analog network, they are being disconnected with the NBN.
However, there are still two ways you can send faxes without a landline: analog faxing (with a twist) and online faxing.
First, you may be able to bring your fax machine onto the NBN using a telephone adapter from an electronics store. Once you have followed the manufacturer’s instructions and configured the device, you’ll need to port your fax number to a telecommunications provider that offers fax services. Then, link your SIP/ VoIP account to your fax number.
Alternatively, you can switch to online faxing (sometimes called 'internet faxing'). Unlike analog faxing, online faxing transmits faxes via the internet - meaning no landlines are required. With online faxing, you would send faxes from your phone, computer or tablet instead of your old fax machine.