The first thing is to try another handset, as that is a probable cause.
A "Like" is always appreciated. If my advice is a solution, please mark it. I'm not a Telstra employee. 50 + years exp. in radio comms., eg mobile phones since 1963, two-way radios, base stations, antennas & pagers. One of my mottos: "Believe in yourself, and you can accomplish anything".