Masayoshi Son isn’t used to taking no for an answer. But the Softbank chief’s pleas for a merger of the third-and fourth-largest U.S. wireless carriers seem to be falling on deaf ears. It is no secret that Son, known to have threatened self-immolation to get his way in the past, wants to combine Sprint, which Softbank acquired last year, with T-Mobile as part of his vision to create a global industry leader.