Source: T-Mobile.

It's easy to see T-Mobile (NASDAQ:TMUS) as the good-time Charlie of the U.S. wireless carrier world. The company's outspoken CEO and connoisseur of four-letter words, John Legere, has helped reshape the wireless space by kicking contracts to the curb, giving out gobs of free data through Music Freedom and Binge On and stealing Sprint's subscriber thunder -- all with a Red Bull in his hand. 

For those watching T-Mobile, it seems like Legere and his company have been hosting a huge customer acquisition party for the past few years, giving out SWAG bags of wireless goodies. And to the carrier's credit, lots of customers have wanted to join in. Over the past year, T-Mobile added 8.3 million net customers (I'm one of them), an increase of more than 14% year over year.

But at times it's hard to believe that T-Mobile's growth party can continue to rage on.

Source: T-Mobile.

T-Mobile's party is fueled by getting more and more people in the door using an onslaught of promotions from its pervasive Un-Carrier events. Over the past few years, the carrier has rolled out offers including: music and video streaming that don't count against customers' monthly data limits, monthly data rollover plans, and generous international roaming and texting options, and has bumped up monthly data allotments at least twice without increasing prices. 

But even the carrier's parent company, Deutsche Telekom (which owns 66% of T-Mobile), believes more money needs to come in from the carrier's customers. 

Deutsche Telekom CEO Tim Hoettges told Re/code last year that, "We have done what we had to do. We had built an infrastructure and this infrastructure had to get utilized and we did that with very aggressive promotions." But Hoettges ultimately concluded that T-Mobile's promotional approach would be unsustainable. 

"The question is always the economics in the long term... and earning appropriate money. You have to earn your money back at one point in time," he said. 

In a way, T-Mobile has started moving in this direction. While average revenue per account (ARPU) fell 3.7% in the third quarter, year over year, average billing per account (ABPA) is "at an all time high" Legere said, up 11% year over year. That's a start, but the carrier needs more customers handing over more money if it wants to continue rolling out new promotions.

What's still unclear, and what could put a damper on T-Mobile's subscriber growth in 2016, is if the carrier will be able to continue the onslaught of Un-Carrier promotions it's become known for. Analysts from Jefferies noted after the Q3 results that it's not time yet to worry about APRU, but said "continued promotional activity should be watched closely." As we begin 2016, it'll be interesting to see if T-Mobile can cut back on expensive Un-Carrier promotions and still grow subscribers. 

Like any college student nearing the end of their time in school, T-Mobile would do well to start looking for a longterm strategy for making money. T-Mobile doesn't have to grow up, it just needs to find a sustainable way to pay for all those subscriber parties it's been throwing.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.