A merger between the third and fourth-largest American carriers would create stronger competition for the two largest, Verizon and AT&T. It could also be seen as weakening the market by reducing the number of big players. The acquisition would have to be cleared by US antitrust regulators.

The Federal Communications Commission (FCC) quashed AT&T's attempt to buy T-Mobile in 2011 for a much greater sum of $39 billion, saying a merger would essentially be anti-competitive. SoftBank's bid could provoke the same ruling from the commission.

SoftBank is offering more than $19 billion

The fact that Sprint is smaller than AT&T may tip the scales in SoftBank's favor, but T-Mobile has been gaining subscribers recently. A merger of Sprint's market share, SoftBank's capital, and T-Mobile's momentum may prove too powerful in the eyes of the FCC — but it looks like SoftBank wants to try anyway. If it doesn't work out, there's always Dish.