Buried on the "Personal" section of the Telstra website is a page entitled "How we keep your personal information anonymous":
We ensure personal information is made anonymous before it’s used by removing data that can identify you, such as your name, address and telephone number. We also use a variety of statistical techniques and operational controls to anonymise data. Anonymising information is one of the tools we use to protect your privacy ... We may share or sell anonymous information to our business partners for research, analysis, marketing, advertising and related uses. That process is carefully controlled so that the information is only used for the stated purpose, and so that individuals are not identified.