Voice services on NBN are provided via the internet. The main difference is that the signal is digital rather than the old service which is analogue.
If you have an existing analogue phone, in most cases it should continue to work on NBN Fibre connections by just plugging it in to the voice port (marked UNI-V) on the modem or NTD box. Your service provider should be able to advise you when you are signing up for a plan if additional equipment is required to connect your phone.
NB some older landline telephone handsets (such as a dial/rotary phone), back-to-base alarm systems, personal response systems (medical alert/emergency call systems) and fax machines may not work over the NBN. You can check with the manufacturer to see if they are compatible.
The other options is a VoIP phone which plugs into the data port (marked UNI-D) port on the back of the modem or NBN box. For these services the service provider will typically provide you with a residential gateway or VoIP phone.
If you are connected via FTTN, FTTB, or HFC your phone service will not work in the event of a power failure. You should consider having another means of making calls in the case of an emergency, like a mobile phone.