America's fastest-growing wireless carrier remains its most popular one -- at least when it comes to full-service brands -- for the second year running.

In addition, the No. 1 player in the space was the only one of the four major carriers that saw its customer satisfaction score improve year over year, according to Market Force's annual study of wireless providers. The customer experience management provider surveyed more than 8,200 consumers, who named T-Mobile (NASDAQ:TMUS) as the country's favorite full-service provider and picked a fast-growing upstart, Consumer Cellular, as the top non-contract provider.

T-Mobile CEO John Legere

T-Mobile has been growing faster than any other major wireless carrier. Image source: T-Mobile.

What did the survey say?

Respondents to the survey were asked to rate their satisfaction with their wireless carrier, along with their likelihood of recommending the brand to others. Their answers were then averaged to create a score on what Market Force calls its "Composite Loyalty Index."

What's perhaps most notable is that none of the four major carriers scored all that well. T-Mobile took top honors with a 43% score, while Verizon (NYSE:VZ) came in second at 37%, AT&T (NYSE:T) took third at 32%, and Sprint (NYSE:S) came in last with 26%.

All of those results were soundly beaten by Consumer Cellular which scored 73%. The smaller player in the wireless space targets the over-50 market by offering low prices on simple phones. It was a repeat winner having scored top marks in 2016 as well.

In addition to its core ranking Market Force also ranked the four major carriers in a number of other categories.

"T-Mobile ranked highest for value, plan flexibility, ease of changing plans, new technology and store customer service, but lowest for frequency of dropped calls," according to the press release. "Verizon was a clear leader for network coverage, call clarity & strength and least amount of dropped calls, while AT&T placed second in all of these categories."

T-Mobile is gaining

These results show that even though T-Mobile has made major strides with its network it still has work to do. It is clear though based on the company's past few years of results that more consumers are willing to give the company a try.

The carrier has added at least one million customers each quarter for the past four years. That's an accomplishment CEO John Legere happily celebrated in his company's Q1 earning release.

"We've been beating up on the competition for over 4 years now while making wireless better for consumers," said Legere. "Q1 was no different with T-Mobile again producing the best customer and financial growth in the industry."

Room to improve

While T-Mobile scored a major win in its category, all four full-service carriers scored less than Consumer Cellular, and at least two other limited-service carriers including third-place Cricket (51%). 

Those numbers suggest that even customer-friendly T-Mobile can't compete with the lower prices being offered by the limited-service players. That may be a cause for concern for all four major carriers because the survey also showed that the number of people using these less-frills, pre-paid phone companies has doubled from 5% to 10%.

Overall, the results of this survey are a victory for T-Mobile and the fact that its numbers got better while AT&T, Verizon, and Sprint all dropped, is encouraging. Still, even the "Un-carrier" has to keep improving or it runs the risk of losing customers to cheaper rivals.

Daniel Kline has no position in any stocks mentioned. The Motley Fool owns shares of and recommends Verizon Communications. The Motley Fool recommends T-Mobile US. The Motley Fool has a disclosure policy.