Shares in Japan\'s Softbank plunged more than 15 percent Friday on news that it was in talks with Sprint Nextel on a potential takeover of the number three US wireless carrier. Softbank shares opened at 2,431 yen on the Tokyo Stock Exchange, down 15.62 percent from Thursday\'s close, as investors fretted about what could be one of Japan\'s biggest-ever overseas acquisitions. The share price was still around the opening level 10 minutes after trade started. Sprint Nextel said Thursday it was talking to Softbank on a possible takeover by Japan\'s third largest mobile carrier. In a statement, Sprint said it \"confirmed that it is currently engaged in discussions with Softbank regarding a potential substantial investment by Softbank in Sprint\". Softbank also confirmed on Friday that talks were going on with Sprint Nextel but noted no decisions had been made yet. The Wall Street Journal said Softbank\'s proposal would include a cash injection of some $12.8 billion in exchange for newly issued shares and an offer to buy additional shares on the secondary market. Sprint Nextel shares surged 14.3 percent to close at $5.76 Thursday on the news. In Tokyo, however, the magnitude of the potential deal weighed on investor sentiment. \"The scale of the Sprint deal is massive, and frankly, difficult to fully dissect at this point,\" Toshiyuki Kanayama, market analyst at Monex, told Dow Jones Newswires.