Japan's NTT DoCoMo said Friday that it will exercise its option to purchase more shares of U.S.-based AT&T Wireless.

The Japanese company previously purchased 16 percent of AT&T Wireless. DoCoMo said in a statement that it's additional purchases will maintain that percentage of ownership. DoCoMo said its decision is contingent on AT&T Wireless' acquisition of TeleCorp PCS, a regional wireless operator in the U.S. That acquisition would occur through a stock exchange, which will increase AT&T Wireless' total number of offered shares. DoCoMo estimated that its additional share acquisition will give it a total of 26.6 million shares of AT&T Wireless' common stock worth about $380 million. Through it's initial buy-in, NTT DoCoMo received one seat on AT&T Wireless' board. Also part of that initial deal, AT&T Wireless agreed to add a GSM/GPRS overlay to its existing TDMA wireless system, a process it already has started. It also is expected that services based on DoCoMo's i-mode technology, which is wildly successful in Japan, will be launched by the American carrier. DoCoMo in the last year has acquired, or attempted to acquire, stakes in a variety of wireless operators around the world. For instance, it recently invested in Dutch operator KPN Mobile, which said recently that it soon will launch i-mode services. However, earlier this week it ended talks with Korea's SK Telecom to acquire a stake in that company.