The European Commission is concerned that Belgian mobile phone operator Mobistar is not obeying regulations passed in June designed to cut the cost of using a mobile phone while roaming abroad, telecoms commissioner Viviane Reding said Thursday.

"There are complaints that the operator hasn't told its subscribers that they can ask to have the eurotariff. It's an issue of transparency," Reding said at a news conference. Mobistar will be fined if it is found to have flouted the roaming law.

The roaming regulations oblige mobile operators within the European Union to offer the so-called eurotariff from July 30, with prices of no more than €0.49 ($0.69) per minute excluding tax for calls made while travelling abroad, and no more than €0.24 per minute for calls received while abroad.

Mobistar is one of a tiny minority of mobile phone operators suspected of having failed to honour the new roaming law, Reding said.

A large majority have made the eurotariff available, she said, adding that operators in Luxembourg, Poland, Germany, the Netherlands and Portugal have gone a step further by making the eurotariff the default setting on phones, rather than an option as initially required under the roaming law.

By Aug. 30, around 200 million E.U. consumers had already switched to the eurotariff, according to a study carried out by the Commission, together with the 27 national telecoms regulators which together comprise the European Regulators Group (ERG), the Commission said.

"Consumers in the European Union have been paying up to 60 percent less for using their mobile phone abroad since this summer," the Commission said.

"The eurotariff is now the standard price offer for roaming in the E.U., as had been the intention of the European legislator," Reding said. "I am glad to see that the move to the eurotariff has gone smoothly," she added.

"I note some remaining problems regarding the transparency of some of the new roaming offers. However, after talks I had today with the ERG and with the European consumer association BEUC, I am convinced that national regulators will take this in hand as soon as possible," Reding said.

The Commission and the ERG are now turning their attention to the markets for text messaging and data transfers, amid concerns that operators are trying to make up for lost roaming revenues by charging more for these services.

Roberto Viola, acting chairman of the ERG, said he doesn't know if this is happening: The ERG is still analysing price data.

"We'll have to wait and see," he said, but added that there are no signs of domestic prices being increased to compensate for the lost roaming revenues.

"National prices have not gone up, despite predictions that they would. Consumer organizations reached the same conclusion," he said. He promised a thorough examination of text and data transfer prices.