T-Mobile has added another 2.1 million customers in the fourth quarter of 2014, capping the end to the best year in the carrier's history, adding 8.3 million customers throughout the year.

CEO John Legere has been an overwhelming force for good at T-Mobile, launching a range of 'Un-Carrier' plans for customers to get more from their contracts. The total T-Mobile customers now sits at 55.018 million, just below Sprint's 55.037 million. Unless Sprint announce a loss of customers in fourth quarter, it will remain this way going into the first quarter of 2015.

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Hitting third place still puts T-Mobile way behind AT&T and Verizon Wireless, both sitting above 100 million customers on contracts. T-Mobile wants to broaden its spectrum in 2015 and add more infrastructure, to make its network even larger. According to Legere, T-Mobile is already the fastest average speed, it just lacks the spectrum AT&T and Verizon Wireless own.

Legere has also confirmed more Un-Carrier events will come in 2015, to liven up the wireless industry even more. At the current rate of growth, T-Mobile could hit 75 million customers by the end of 2015, and even higher if it continues to take customers away from AT&T and Verizon Wireless.

However, some investors are concerned the amount of new features on the Simple Customers package will put T-Mobile at an operating loss. For now, this does not seem to be an issue, with T-Mobile still reporting profits.

AT&T recently announced its own data stash program, but Legere and other critics claim the plan does not provide the same quality as T-Mobile's Data Stash, only securing data for two months, instead of one year.

TagsT-Mobile, U.S. carriers, John Legere