OSAKA -- NTT West Corp. said Monday that phone services were temporarily disrupted after a massive number of calls were placed by a firm suspected of operating a "wangiri" callback scam.

The problem occurred throughout Osaka Prefecture and in Amagasaki, in neighboring Hyogo Prefecture.

NTT West, a regional unit of Nippon Telegraph and Telephone Corp., halted the unnamed firm's telephone services after complaints about the disruption began pouring in around 10 a.m.

NTT West officials said about 5.16 million lines in the area were affected. The telephone service returned to normal at 2:44 p.m., NTT West officials added.

It is the first time an NTT group firm has suspended the lines of a specific client for causing such a problem, the officials said.

The same firm, based in Osaka's Kita Ward, was responsible for widespread disruptions in the same area on July 15, they said. The company cannot be named because of communication privacy rules, the officials said.

In the wangiri scheme, people, mostly mobile phone owners, receive an incoming call that only rings once. But the number of the calling party remains on the phone's screen, often inducing users to return the call.

Callers to the number are directed to a taped phone sex message or information about adult entertainment and are sometimes charged huge fees.

To avoid a system collapse, NTT West had switchboards serving the company operate at only 50 percent capacity until it decided to shut down the firm's telephone service at 10:59 a.m., the officials said.

On July 15, the Osaka firm used 216 lines and made more than 4,000 random calls every three minutes for several hours, affecting 5.16 million user lines.

Since that incident, NTT West has monitored the firm's lines, the officials said.

But NTT officials said the provider has no effective way to combat such schemes.

"Effective measures must be considered by the government, including revision of legislation," an NTT official said.

Telecom ministry officials, however, said government regulation is unlikely for the moment.

"Police can lodge charges if phone lines are used for crime. But at this stage, it is a problem for telephone companies and clients," said an official of the Public Management, Home Affairs, Posts and Telecommunications Ministry.