Sprint Nextel confirmed that it is in discussions with Softbank, a Japanese telecommunications firm, over a “possible substantial investment” in the U.S. carrier.

In a statement, the company said that the talks could “involve a change of control of Sprint,” but that discussions were not final.

The firm said that it will not comment further on the discussions unless the companies reach an agreement. A report from the Wall Street Journal indicated that it was not clear if Softbank was planning to buy all of Sprint, or considering a two-thirds stake in the company.

Softbank, which is run by billionaire Masayoshi Son, is the third-largest carrier in Japan. As Bloomberg reported, the firm recently announced it was purchasing a smaller Japanese competitor, eAccess, to build out its high-speed LTE network.

Reports surfaced Thursday that Sprint, the third-largest carrier in the United States, was considering an acquisition bid of at least $12 billion from the Japanese firm. The news comes a week after the fourth-and fifth-largest U.S. carriers, T-Mobile and Metro PCS, announced a merger proposal had gotten the approval of both companies’ boards.

Sprint shares rose dramatically in Monday trading, up 17 percent to $5.93 per share as of 11:30 a.m. Eastern.

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