I am in Ballina, northern NSW, and I have two mobiles, one an I phone and the other an OPPO.
Both phones are having the same problem. I am in the middle of a phone call, (whether I am calling out or receiving makes no difference) and suddenly it sounds as though I am underwater, with loud gurgling sounds. The call must be discontinued and I have to cal the person back over and over to complete the conversation.
This didn't happen until around November 2018.
What is going on with the Optus network in the area?