Sounds like outgoing call baring has been activated on your line. Call 150 or do an online fault test on your account page.
Einstein may pick up the issue and reprovisio the line.
I know some systems would activate a soft disconnect of a landlines that has not had a voltage change in a set period of time. A voltage change occurs when the handset is lifted and a dial tone produced or to answer an incoming call.
Feel free to click on the thumbs up thingy and kudos thing. It really helps me to feel important and boosts my ego.
Seriously though, I'm happy to help.