TOKYO, Sept 21 -- Nippon Telegraph and Telephone (NTT) Corp., which owns 30 percent of Japan's fiber optic networks, will allow rival carriers to use its fiber optic network. The Japanese government's recent attempt to unfasten NTT's hold on the telecommunications market prompted NTT to quit its practice of leasing lines and switchboards as a package deal even though clients may not need the latter, said the Nihon Keizai Shimbun financial daily. Customers will now be able to lease optical cables by themselves, and NTT will accept joint use of a single cable as well. The decision was made after a Monday meeting between the presidents of the holding company and its two regional carriers, NTT East and NTT West.