What is the fine-grained topic of the following text: "NTT DoCoMo Inc., a mobile phone unit of telecoms giant Nippon Telegraph and Telephone Corp., said it had 6.92 million subscribers to its Internet service known as i-mode as of Sunday, and the number of subscribers was increasing by 20,000 a day. Only return the topic.
NTT DoCoMo Inc.'s increasing number of i-mode Internet service subscribers