T-Mobile (NYSE:TMUS) is making good on its promise to expand its retail footprint to leverage the build-out of its LTE network, according to Wave7 Research. But the carrier's effort to sell postpaid devices through Walmart outlets is coming to an end.

The nation's third-largest carrier has aggressively expanded its LTE network in suburban and rural areas over the last two years, and it has recently opened 10-15 new stores to tap those markets. Wave7 said its social media checks indicate new stores have opened in small and mid-sized markets in states including Colorado, Iowa, Michigan, Arkansas, Wisconsin, Texas, and North and South Carolina.

"The carrier is also opening some stores in established markets, such as Orlando and Flint," Wave7 wrote. "The carrier already had a store on Times Square in New York, but a new, much more highly visible store opened there in January."

T-Mobile recently spent more than $1 billion to acquire 700 MHz A Block licenses to expand its network in previously uncovered areas in Utah, New Mexico, the Southeast and elsewhere. And it's moving hastily to add as many as 400 retail outlets to sell devices more effectively and raise brand awareness in those markets.

"We now cover 305 million people in the U.S. with our 4G LTE footprint, and we think right now we have the opportunity to expand our distribution into another 30 to 40 million POPs here in the U.S. where we have zero penetration," CFO Braxton Carter said last month. "The ecosystem's there."

That existing ecosystem notwithstanding, Walmart's six-month pilot of selling T-Mobile's postpaid services through 600 stores appears to have flopped. The retailer has halted the program, leaving Verizon and AT&T as the only carriers to have a postpaid presence in most Walmart stores.

The massive retail chain sells isn't a major player in postpaid phones; the overwhelming majority of its phone sales are prepaid. Wave7 reported in December that an effort to sell Sprint's (NYSE: S) postpaid phones at Walmart was coming to an end, although the carrier denied that.

Walmart had turned to T-Mobile as a postpaid alternative late last year as Sprint's services were phased out. Sprint denied its presence at Walmart was on the wane, however.

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