Softbank owns 80% of Sprint (S).
Son has promised a price war if the merger is allowed to go through.
T-Mobile has experienced a resurrection of sorts after waging its own price war against the top three U.S. mobile carriers by offering low-cost no contract plans to its customers and even offering a deal to pay contract termination fees for consumers who want to switch to T-Mobile service plans.
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These moves have prompted industry leader AT&T (T) to respond by abandoning its premium pricing strategy in favor of low-cost no-contract plans similar to T-Mobile's.
Son insists that the only way to increase competition in the U.S. market, and therefore lower prices, is by allowing third-place Sprint to merge with fourth-place T-Mobile in an effort to place more competitive bids at broadband spectrum auctions.
T-Mobile was up 2.8% in afternoon trading to $31.83.