By Maggie Lu Yueyang SYDNEY (Reuters) – Australias biggest phone company Telstra Corporation Ltd has agreed to sell 70 percent of its directories unit Sensis, offloading a struggling business challenged by digital transition for less than the market had anticipated. The sale of Sensis, following Telstras sale of Hong Kong mobile phone business last month, would further boost the telco giants cash war chest to invest in new growth businesses and expand its mobile network. Telstra said on Monday that it had agreed to sell the Sensis stake to Platinum Equity, a U.S.-based private equity firm, for A$454 million ($407.17 million). Telstra will retain 30 percent of Sensis, a business valued at A$649 million, it added.