NBN Compatibility

 All phone systems are compatible with the NBN either directly or indirectly

PSTN:

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

ISDN:

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

VoIP/SIP:

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.

With the NBN supposedly fully rolled out this is mostly no longer an issue. there are some stragglers out there and for you guys. you are definitely compatible. only the most ancient of hardware isn't compatible and it most likely all got thrown at the nearest wall when it failed to be compatible with phone banking.