The French company doesn't want to pay more on its own, but is rounding up allies to help persuade Deutsche Telekom to sell its controlling interest in the US carrier.
A new salvo in the wireless price war cuts the price of shared data on two lines by 29 percent.
French telecom provider says T-Mobile and parent company Deutsche Telekom refused to entertain its new offer.
The company will let new and existing postpaid customers add an LTE slate to their existing plans and get nearly 1.2GB of free data through the end of the year.
The carrier attracted 61,000 new customers in total in the fourth quarter, though sales fell by 5.2 percent from a year ago.
SoftBank CEO Masayoshi Son will reportedly argue that having a strong third mobile carrier will help even the playing field for in-home broadband.
If regulators borked the $39 billion merger of the No. 2 and No. 4 wireless carriers, is the $45 billion combo of the two biggest cable companies doomed? The competitive dangers are very different.
FCC Chairman Tom Wheeler is the latest regulator to pour cold water on a potential merger between the third- and fourth-largest wireless carrier.
The carrier's Mobile Money program is designed to go after customers who haven't traditionally had bank accounts.
T-Mobile USA may soon offer family plan consumers an incentive to ditch their current providers for T-Mobile as part of its "UnCarrier 4.0" strategy, according to a tweet from CEO John Legere.