Called today and son answered it. Indian voice - lady- said she was from Telstra and wanted to inform us that someone outside of our house was using our internet. Thank goodness he hung up.
I have a silent unlisted number and only a handful of people know this number. So much for Telstra security. Have called Telstra Unwanted Calls and they will block the number for you. Only problem is it blocks all calls from overseas coming in.