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Sprint (S) reported its fiscal first-quarter 2016 results this week, and arguably the most important numbers were its 173,000 postpaid net phone additions.

This time last year, Sprint reported it had lost 12,000 net postpaid customers (typically a more sought-after customer because they have higher credit scores and are less likely to jump around from carrier to carrier). Sprint's been fighting an uphill battle to regain customers as rival T-Mobile (TMUS 0.50%) has rocketed past it with stellar customer gains over the past few years. But do Sprint's latest customer additions prove the carrier can finally take on its competitors? 

Close but no cigar

Sprint should be applauded for its recent customer gains. About two years ago, the company reported a net loss of postpaid phone subscribers of 205,000 -- in just one quarter.

Sprint's been able to bring customers back by offering discounted pricing, along with a wide range of options. It's the only carrier to offer no-contract plans, leases, and two-year contracts right now. It has also proved that it can add customers and keep them at the same time. Its churn rate (the rate at which customers leave to go to another carrier) for the first quarter was just 1.39%, the lowest in the company's history. 

The carrier's network (while is still far from perfect) is getting better as well. RootMetrics' network performance report ranked Sprint in third place, behind Verizon and AT&T, for the second half of 2015. The report noted that Sprint had improved its network in all key areas: metro, state, and countrywide level.

All of the positive news has helped push Sprint's stock up an amazing 63% so far this year (including a pop after the latest quarterly earnings report). Despite all that, though, there are still a few things Sprint investors should keep in mind. 

First, the carrier posted net losses of $302 million this quarter, much higher than the $20 million in losses from first-quarter 2015. Some of those losses came from a $113 million contract charge that Sprint had to pay out after ending its partnership with Ntelos. 

But some of those losses are also likely coming from its discounted prices for wireless plans and promotional offers. This isn't all that bad, but eventually Sprint will have to scale back its promotions and increase prices in order to earn some positive revenue. 

Second, while Sprint's network surpasses T-Mobile's in the RootMetrics report, the latter constantly averages higher network speeds than Sprint and is improving all the time. This means that any network lead Sprint has right now could be overtaken by T-Mobile soon. 

And finally, an American Customer Satisfaction Index (ACSI) report released last month showed that T-Mobile topped the list among the nation's biggest wireless carriers for overall customer satisfaction. Where was Sprint? Dead last. 

Sprint investors should certainly be optimistic about the carrier's future, but I don't think they should look past the company's revenue challenges or T-Mobile's rising position. Sprint may be taking a page or two out of T-Mobile's book on how to grow customers with cheap prices and freebies, but the carrier will need to see sustained customer growth and higher revenue before this stock looks like a solid long-term investment.