The Philippine Long Distance Telephone Co. is investing $100 million in an undersea fiber-optic cable system called Synchronous Digital Hierarchy-Wave-Division Multiplex. Phase 1, expected to be completed by March 1977, is expected to increase the company`s capacity by 20%. The company has also signed a memorandum of understanding with AT&T Corp. and Kokusai Denshin Denwa of Japan to conduct a construction-feasibility study.