At times, T-Mobile (NASDAQ:TMUS) CEO John Legere seems a bit like a villainous 1980s pro wrestling manager making absurd boasts about what his company is going to do to the competition in the same way Bobby "The Brain" Heenan would tell Hulk Hogan how badly "King Kong" Bundy was planning on hurting him.

In recent months, Legere has veered from the plausible -- that his company would pass Sprint (NYSE:S) for third among the big four mobile carriers -- to the harder to swallow, that T-Mobile has set its sights on passing No. 2 AT&T (NYSE:T) in total subscribers. He talked about that during the company's Sept. 10 "Un-Carrier" event (link opens a video), and he repeated the boast on Twitter. 

Catching Sprint is one thing, as the No. 3 carrier has only a slight lead on T-Mobile, but passing AT&T for No. 2 behind Verizon (NYSE:VZ) is an incredibly tall order. 

T Mobile

John Legere at T-Mobile's "Un-Carrier" event. Source: T-Mobile. 

How big is the gap between the two?
T-Mobile closed its second quarter with over 50 million subscribers, while Sprint claimed over 54 million in its most recent financial release. AT&T did not list its total subscriber number with its first-quarter financial release, but GeekWire puts its total at "over 117 million subscribers." 

During their most recently reported quarters, T-Mobile added 1.5 million new subscribers, Sprint lost 220,000, and AT&T added just over a million. If those numbers stay relatively the same moving forward, T-Mobile would make up its 4 million subscriber deficit with Sprint in under nine months. Its 67 million-customer gap with AT&T would take a little longer. If T-Mobile closes the gap by 2 million customers each year, his company should pull even in 33.5 years.

Legere would likely point to the fact that in August, T-Mobile gained 2.75 million customers, the largest number of monthly subscriber additions in its history. If those numbers hold up, the company would be adding 8.25 million customers a quarter, or 33 million a year, while AT&T adds 4 million. Under that scenario, T-Mobile would pass AT&T in a little over two years.

Of course, assuming T-Mobile will continually repeat its best month ever is something nobody -- outside of perhaps Legere -- is likely to predict. The company has added over 1 million subscribers for each of the last five quarters, so it's reasonable to predict that pattern will continue. For now, at least, it appears its August success is an anomaly tied to its aggressive pricing strategies and Sprint's woes.

With Legere in charge, T-Mobile may be able to produce a month of tremendous growth every now and then, cutting into its 33-year march to catch AT&T, but it's unreasonable to think it's going to regularly equal its stupendous August.

Could AT&T stumble?
So far, most of T-Mobile's gains have come at the expense of Sprint. During the conference, Legere said that at the peak of its "Contract Freedom" ad campaign, the ratio of subscribers moving from Sprint to T-Mobile was 17 to 1. That number has fallen to around 2.to 1, he added, so it makes sense for the CEO to pick a new enemy to focus on.

AT&T is next on the list once T-Mobile passes Sprint and it makes an inviting target. The No. 2 carrier has tweaked its offerings as T-Mobile and now Sprint have begun aggressively going after new customers by offering to buy them out of their contracts. Still, AT&T's plans are more expensive than T-Mobile's in general -- in most cases by a fair amount -- which Legere has gleefully pointed out on Twitter.

For T-Mobile to make significant strides, it needs AT&T to not really worry about the threat his company represents. Verizon and AT&T have little reason to change, he told CNET, because their businesses are so profitable. 

"If they're happy, I'm happy," he said. "If they think nothing is going on, have a party."

At some point, though, even the most content, lumbering giant is likely to wake up, and it's not like T-Mobile can sneak its way to covering a 67 million-customer gap. Still, in setting his sights high, Legere has put his company and the industry on notice. T-Mobile may not be a credible threat to AT&T for the next few years, but it could win customers at its bigger rival's expense, and that's more the point than any bravado about taking over second place.

Daniel Kline has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.