In the tech and telecom world, it's pretty hard not to notice T-Mobile (NASDAQ:TMUS). The carrier has aggressively pursued customers over the past few years, while building a strong network that's catching up with its rivals'. With its blitz of Un-Carrier events, T-Mobile has managed to garner 61 million customers, usurp Sprint (NYSE:S) as the third-largest U.S. wireless carrier, and usher in an era of contract-free smartphones in the U.S.

But with all its influence in the wireless carrier space, T-Mobile is spending $4 billion to $5 billion each year to battle competitors, and even its parent company, Deutsche Telekom, doesn't believe the carrier's aggressive customer acquisition approach will last much longer.

Click on the slideshow below to find out what T-Mobile's doing right in the wireless market, what it needs to change, and how the carrier can sustain its business.