I am in a gated community and visitor admittance is by a mobile phone SIM from the gate to our NBN landline. We press * on the handset to open the gate.
Since installation of the NBN the phone does not receive a call from the gate, but accepts all others - both landline and mobile. We have been here for 14 years and this is the first problem we have faced.
We have a Panasonic KX-TGA431AL with 2 handsets.
At present we cannot admit any visitors, so this is urgent.
This sounds like a complicated dilemma for you, sorry to hear that.
I would suggest contacting the community corporate managers, or whoever is in charge of the gate maintenance and operation of the gate to see if they can provide some immediate help.
We cant comment on how this all works, but its possible your older phone service ran through a control box inside your home, and since the NBN has been activated, something has changed in the setup.