But hours from going public, the buyer–Sprint–pulled out. At the last minute, the Kansas City-based carrier’s board voted down what would have been one of the more significant merger deals of 2012.

“It’s one of the most astounding things that has happened in M&A–you just don’t do business that way,” Braxton Carter, the former vice chairman and chief financial officer of MetroPCS, recalled this week.

That board didn’t last long. Within months, Japanese billionaire Masayoshi Son bought control of Sprint and over the next few years replaced all but one member of the board: former Continental Airlines CEO Gordon Bethune. Sprint didn’t respond to a request for comment about the failed deal, which took place more that two years before current CEO Marcelo Claure took the helm.

But Sprint’s rejection opened the door to another deal.

A few months later, MetroPCS founder Roger Linquist got a call from René Obermann, then the CEO of Deutsche Telekom, to discuss being acquired by DT’s U.S. wireless unit, T-Mobile. Obermann was dealing with the aftermath of AT&T’s blocked effort to acquire T-Mobile. As part of a termination fee, AT&T had to give T-Mobile $3 billion in cash and additional spectrum licenses. So after trying to dump T-Mobile, Obermann now saw a chance to build up its reinvigorated U.S. unit. Within weeks, the companies had a deal.

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The result was one of the more surprisingly successful mergers in telecommunications history. Passing the fourth anniversary of the deal this week on May 1, T-Mobile’s stock had more than quadrupled, while competitor AT&T’s (T) stock price gained just 5% over the same four years, Verizon (VZ) lost 15%, and Sprint (S) rose 29%. And the still-active MetroPCS prepaid brand has nearly doubled its customer base, from nine million to almost 18 million.

“It’s been a hell of a ride,” Carter, who joined T-Mobile (TMUS) as CFO when the acquisition closed, tells Fortune. Not afraid to take a stab at a rival, Carter says that T-Mobile’s big gain “was Sprint’s opportunity that they let slide through their fingers.”

Bigger Win

By almost any measure, the MetroPCS deal has been an even bigger winner than T-Mobile CEO John Legere and his team promised investors back in 2013. Legere relied on his new acquisition to market to so-called value customers—those with less income or weaker credit who pre-pay for service—while moving the T-Mobile brand up market to challenge market leaders AT&T and Verizon head-on for more lucrative customers who get billed at the end of every month. While unifying the network infrastructure, the brands stayed separate with separate marketing efforts and retail chains.

“This isn’t a company that thinks about internal competition,” says chief operating officer Mike Sievert. “We think about external competition. So we’ve gone with two great brands.”

When the deal was announced, T-Mobile promised annual revenue growth of 3% to 5%, but has reported much stronger increases of 16% last year, 8% in 2015, and 13% in 2014. Total synergies for the deal—Wall Street-speak for cost savings—hit $9 billion, more than the $6 billion to $7 billion originally promised.

And annual savings of $1.5 billion-a-year were expected within four years from combining operations and decommissioning MetroPCS’s incompatible network by the end of 2016. But under T-Mobile’s network chief Neville Ray, T-Mobile shut down most of the MetroPCS’s CDMA equipment within 12 months and finished the full migration of all MetroPCS customers by July 2015. It hit the $1.5 billion annual savings target a year ahead of schedule.

And Ray also cleverly combined negotiations to shutter leased cell sites from MetroPCS with talks to expand T-Mobile’s footprint on cell towers. As he dumped 10,000 MetroPCS sites, Ray was adding up to 8,000 spots on towers for T-Mobile, mostly for spectrum rights that had just been acquired in the valuable 700 MHz band.

“We’re very tough, hard negotiators here,” Ray says. “We were able to translate a lot of those ongoing legacy costs on the sites that we needed to decommission to future growth needs that we had in the business.”

Wall Street, which was initially skeptical of the MetroPCS deal, has come around, too. “The PCS deal is now the template for almost any proposed telecom merger,” says longtime industry analyst Craig Moffett.

Given the larger-than-promised cost savings T-Mobile collected by the fourth anniversary, marathon runner Legere is ready to take a victory lap this week. “Billions more in synergies delivered earlier than forecast and today we have the fastest growing brands in both postpaid and prepaid,” he crows.

More Mergers

It’s a timely boast, however, as the wireless business looks headed for another huge round of consolidation. With customer growth in the overall industry slowing and a fierce price war raging over unlimited data plans, the carriers are looking for deals to enhance their prospects. And with Donald Trump in the White House and Republicans in control of Congress, the amount of regulatory scrutiny on deals may be headed for an all-time low in the modern U.S. economy.

Sprint tried to acquire T-Mobile in the past. But having now surpassed its rival in number of subscribers and stock market value, T-Mobile may be in the driver’s seat. Neither carrier has been commenting directly on a possible deal. Legere told Fortune last month that his company is in a “formidable position” for M&A and he’s ready to talk to Sprint. “I look forward to being able to hear their opinion directly at some point,” he said.

But Sprint CEO Claure sounded less eager to deal when he spoke with reporters on May 3 after the company’s latest earnings report. “We’re in no hurry to jump into anything, and we’re going to be very diligent in evaluating every opportunity,” he said.

T-Mobile gets credit for a “deftly handled integration process” with MetroPCS, says analyst Jonathan Chaplin of New Street Research. But Sprint is vastly larger, with about seven times as many subscribers as MetroPCS had when it was acquired, he points out. “Sprint would be a much bigger undertaking,” Chaplin says. “Neville Ray and his team have done a superb job so far, but it won’t be a walk in the park.”

Technological developments could make the combination simpler, according to Walt Piecyk at BTIG Research. Many phones can now operate on multiple spectrum bands, meaning customers might not have to upgrade their devices to be able to access both company’s airwaves. “Integration should be easier with the integration of more spectrum bands and technologies into common phones,” Piecyk says.

Expect CEO Legere to crow a lot more as he heads out to make another deal likely very soon.