NBN to PBX office phone systems - help !

We have NBN at our office and they tell me that come October 5th, the Telstra copper lines will be pulled out and we have to have phone through NBN.

We have a ten year old Commander PBX with five incoming lines and 12 extensions throughout the office and it works really very well, but no-one can tell me if we can use it somehow with NBN.

I have tried our phone provider, our NBN provider and Telstra, and can’t tell me anything expect send me emails about phone plans.

I recall someone on Blitz, had messaged me yanks ago about this, but like everything, I just left it.

Can anyone offer any advice. Ideally if there is some piece of hardware that goes between my existing system and the NBN lines, so we can keep our phone numbers and use the PBX. I don’t really have the money for a new phone system,

Help !