Nikkei's sources indicate that SoftBank board members decided to call the talks off on Monday, after a Friday meeting where executives determined "the company would not give up control." SoftBank is reportedly expected to propose to Deutsche Telekom on Tuesday that they end negotiations.

Whether that would get Deutsche Telekom to change its mind about the ownership structure is not clear, and we've reached out to Sprint and T-Mobile for comment on this story. Meanwhile, it appears the merger of America's third- and fourth-largest carriers remains in limbo.