NTT DoCoMo Inc. plans to set a monthly flat rate of about 3,900 yen for unlimited third-generation cell phone data transmissions, NTT DoCoMo sources said Wednesday.

The fee is about 300 yen less than the comparable data transmission service offered by KDDI Corp., NTT DoCoMo’s main rival in the fiercely competitive mobile phone market.

NTT DoCoMo plans to introduce the flat data transmission rate as early as June after putting the final touches on the service, the sources said.

The new service will be available to subscribers of 3G FOMA cell phone services, which allow users unlimited use of packet switching communications to download content such as games and ring tones in addition to e-mail and the Internet.

Subscribers will have to pay a basic fee and fees for voice transmissions separately.

NTT DoCoMo lost its status as the No. 1 mobile phone company in Japan in terms of net increases in subscribers last year to KDDI, which provides its mobile phone services under the au brand.

KDDI’s data roaming KDDI Corp. and affiliate Okinawa Cellular Telephone Co. said Wednesday they will start offering in early May an international data roaming service linking Japan and South Korea on the basis of au brand mobile phones.

The service, to be provided in conjunction with SK Telecom Co. of South Korea, will enable mobile users to exchange e-mail and other data via the Internet in addition to making phone calls, the firms said.

KDDI and Okinawa Cellular have already offered an international mobile roaming service called Global Passport, but it is only for phone calls.

The two companies plan to expand the international data roaming service to cover other parts of the world, they said.

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