Last year may very well go down in history as the year that T-Mobile turned the corner. Fresh off a failed takeover bid from AT&T, the struggling wireless provider broke all the rules in 2013 with its UnCarrier strategy and forced change on the industry. It also added 4.4 million new subscribers in the process.

Over the course of 2013, T-Mobile delivered service plans with no annual contract, an early upgrade program that more than 3.6 million customers are now using, rolled out unlimited data and texting worldwide, offered free tablet data for life and just last month, agreed to pay early termination fees in order to get customers to switch to their network.

Some of these changes remain exclusive to T-Mobile while others, like the early upgrade plan, have forced the competition to roll out similar programs to keep pace.

In the fourth quarter alone, T-Mobile added more than 1.6 million customers - a "completely knockout" according to CEO John Legere. It marked the third quarter in a row that the company added more than a million new subscribers.

While they may be winning in terms of adding new customers, they're doing so at a cost. T-Mobile reported a loss of $20 million during the fourth quarter, up from just $8 million during the same period a year ago. As you likely guessed, the losses are primarily a result of spending more on promotions to attract new subscribers.