NTT East Corp. and NTT West Corp. plan to stiffen the terms of their telephone service contracts to prevent phone callback scams known as "wangiri" from paralyzing their networks, company officials said Thursday.

The boards of the two regional Nippon Telegraph and Telephone Corp. carriers have approved the new terms, the officials said.

The two NTT subsidiaries will next ask the Public Management, Home Affairs, Posts and Telecommunications Ministry for approval.