Counterpoint Technology Market Research reported this week that T-Mobile was the third-largest U.S. purchaser of smartphones in the first quarter, overtaking Sprint. T-Mobile bought 6 million smartphones for itself and its MVNOs during the period, according to Counterpoint analyst Neil Shah, outpacing Sprint’s 5 million units.

T-Mobile recently posted its best quarter ever, adding 2.4 million subscribers to claim the lion’s share of the U.S. market’s core subscriber growth. T-Mo has made big strides with its infrastructure lately, too: My colleague Kevin Fitchard reported this week that it launched voice-over-LTE in Seattle on three handsets, and last month Fitchard documented how it is using a technology dubbed 4×2 MIMO to improve signal strength and connection speeds on its LTE network.

T-Mobile’s increasing momentum is particularly noteworthy in light of recent remarks from Sprint CEO Dan Hesse, who continues to lobby in favor of a rumored tie-up between the nation’s third- and fourth-largest operators: “A stronger No. 3 will get one and two to react more aggressively so everybody benefits,” Hesse told Bloomberg Television earlier this month. “If you are smaller, the big two do not react as aggressively.”

Sprint’s 54.6 million customers outnumber T-Mobile’s users by roughly 5.5 million, and T-Mobile’s losses are likely to continue due in part to its growing smartphone purchases. At this rate, though, it won’t be long before T-Mobile overtakes Sprint as the third-largest operator in the U.S. And while T-Mo is a prime target for an acquisition or merger by any of a handful of suitors — from Sprint to Dish Network to Comcast — that growth will increasingly force AT&T and Verizon to “react aggressively.”