NEW DELHI: Indian police investigators quizzed telecoms tycoon Anil Ambani on Wednesday over a suspected mobile licensing fraud that robbed the government of billions of dollars.

Ambani's Reliance ADA group said the appearance by the billionaire at the Central Bureau of Investigation (CBI) headquarters in New Delhi was voluntary and that he had not been summoned.

Ambani, India's fourth richest man with an estimated worth of $13.7 billion, according to Forbes.com, met CBI officials "to clarify ongoing issues, relating to telecom matters for the years 2001 to 2010," the group said.

There was no immediate comment from the CBI.

Indian television channels showed Ambani, who controls the Reliance ADA Group, leaving the CBI offices surrounded by television crews.

Reliance ADA Group, which includes the country's second-largest second-largest mobile phone company Reliance Communications, has denied any wrongdoing in connection with the second-generation (2G) licensing scandal.

Reliance ADA Group said last weekend that police investigators had questioned several of its officials over alleged irregularities in the awarding of 2008 2G mobile phone licences.

The meeting between CBI officials and Ambani marked the latest development in a probe into the government's sale of the 2G licences at knockdown prices, which has become one of the biggest corruption cases in India's history.

The national auditor said earlier that irregularities in mobile licence and spectrum allocation cost the public exchequer up to $40 billion in lost revenues.

The CBI is looking into companies which may have benefited from suspected rigging of licence bidding rules under then telecom minister A. Raja.

Raja was arrested with two former aides earlier this month in the scandal that has tarnished the reputation of the Congress government led by Premier Manmohan Singh.

The police agency has not alleged that any of the Reliance ADA Group firms profited from the scam.

It has, however, named India's Swan Telecom -- now known as Etisalat DB Telecom -- and real estate company Unitech as allegedly having benefited.

The police also suggested that Swan Telecom, which got its licence in 2008, appeared to be acting as a "front company" for Reliance Telecom when it applied for the licence.

However, Reliance ADA Group said on Wednesday no Reliance group company or affiliate "held even a single share in Swan Telecom Ltd., at the time of granting of the 2G license".