Hi @richardszymura Welcome to the TPG community, Uniden's website says that their fixed line products have been tested and are compatible with NBN services so that shouldn't be a problem, have you set the time and date correctly? The answering function won't work if this isn't done as the time stamp won't be correct.
Log into your account and check the settings on your NBN digital voice service control panel, make sure there are no call diversions in the settings, can you actually answer the incoming call?
I don't believe TPG has a voicemail option on NBN residential bundles, have you tried a reset all settings on the cordless system? Try that and set the minimum rings before answer option and see what happens, ensure you're Uniden is plugged into the phone 1 socket on the modem also.
I would suggest taking your base and plugging it into a friend or relatives place to see if it will answer there, ideally a modem on a working NBN service or even just a PSTN line just for curiosity, then if possible borrow someone else's answering machine/cordless base and plug it into your modem.
I'm assuming that there is nothing else plugged in parallel with your Uniden unit to your modem such as another fixed phone?
my mum had same probrem on the answering settings turn time dowm to how many rings my mums was set to six and would not work so i set it to three rings then it answer machines kicks in yes i know it a pain if you at home but theres should be a button on it to turn of and on when home