If you need an answer in a hurry, YES, your phone system is compatible with the NBN
It may need some modification to be 100% compatible. but almost every phone system sold is compatible.
We know of certain well known monopoly backed retailers distributing misleading information regarding NBN compatibility. Any area getting upgraded to fibre services tends to find these sales people informing anyone who will give them the time of day that their phone system, regardless of age or brand isn't NBN compatible.
This is untrue, the oldest phone systems on the market are compatible to a degree. Ideally for the NBN you may want to switch to VoIP lines as this will work out cheaper long term.
DO NOT misinterpret this message as ALL phone systems are 100% compatible AS IS, many will need changes, upgrades, downgrades or reprogramming but only in extreme cases will you NEED a new phone system.
If you had regular Telstra lines previously then your service provider can supply these via sip routers or IAD's (Integrated Access Device) or Voice Gateways
if you had telstra digital phone lines previously as either basic rate ISDN or primary rate ISDN, an ISDN IAD can be used by your carrier to supply the ISDN services. however Telstra currently is allowed to keep running these services on NBNco's copper network
GSM, 3G or 4G:
If you had mobile phone lines previously... nothing changes. you still do
These lines are not just compatible but are the only line type that is directly compatible. The rest will be converted into VoIP by IAD's and ATA's, VoIP has all of the benefits and when installed correctly none of the negatives.