My Brother 515 phone/fax does not have dialtone now I am connected to the NBN even when directly connected to the OPTUS modem. I have connected my landline to the EXTernal port on this machine and it works fine. Brother did a factory reset and say no problem. I have tried to follow the solutions offered on the forum without success. Use fax every day.
Fax is not supported on the NBNCo service at all.
If you aren't getting dial tone when you plug your phone into the Optus modem, try a regular home phone first. Now, you need to know that in some cases, the fax "will work" but not always. Because Fax was never designed to work over IP.