In a move to widen the appeal of its new broadband mobile phone service, Japan's NTT DoCoMo plans to offer an international roaming service in over 50 countries beginning June 1.The World Wing international roaming service targets users of DoCoMo's new Foma handsets based on 3G mobile broadband technology, the Japanese mobile operator said Monday in a statement.When traveling outside Japan, Foma subscribers can make and receive calls by inserting a stamp-sized smart card that comes with their 3G handsets.DoCoMo will charge no monthly fee for the roaming service, the company said. Telephone call costs will vary depending on country, it said, without providing any detailsDoCoMo will initially offer the service in 53 countries but plans to eventually cover 72 countries, it said.Whether Foma subscribers will flock to the Japanese operator's new roaming offer remains to be seen. The service focuses on voice rather than the multimedia mobile data that Japanese mobile phone users are used to with their i-mode handsets. The cost of roaming calls could also be an issue.In Europe, where international roaming is popular, calls made outside the subscriber's home market carry a surcharge that can be as high as 15%. The European Union has been putting pressure on mobile operators in the region to lower their international roaming fees.