Update: Problem appears to have been solved. Today 12 October 2016, after about a week, a Telstra tech arrived at our front door and inspected the wiring inside front gate and house. He said that everywhere outside our house checked out, so it must be inside. In fact, it seems that our numerous long telephone extension cords were too old. So we probably cannot blame Telstra this time. We replaced the old cords and the problem seems to have gone away.