Welcome to TPG Community!
You are correct, the PABX will not work for the VoIP phone that is bundled with your TPG NBN, since the settings is embedded in the firmware of the TPG provided modem.
We were able to locate an account using your community details and the phone status seems to be registered. Did you connect a handset directly to the Phone port of the modem? Have you tried both phone ports?
Thank you for the information.
In order to check on this issue further, we'll escalate the case to our Engineering team for further investigation.
If there's a need to replace the modem, then they'll be the one to process that for you.
An assigned Engineer will be in touch with you within 24 to 48 hours using the number that we have on file.