T-Mobile (NASDAQ:TMUS) and Sprint (NYSE:S) have been the aggressors in the mobile phone business using aggressive promotions and pricing to attempt to win market share from category leaders AT&T (NYSE:T) and Verizon (NYSE:VZ).

The number three and four carriers (which is which has become too close to call) have been following the classic underdog strategy of calling out their more successful competitors. T-Mobile's CEO John Legere has been the most aggressive user of this strategy -- often acting like a 1980s pro wrestling manager -- but Sprint, under new CEO Marcelo Claure has followed along the same pattern.

Legere, however, has set the tone with a relentless media and social media campaign designed to undermine AT&T and Verizon.

Source: Twitter

That posturing and both T-Mobile and Sprint offering cheaper service, often with more perks (like Legere's company never charging overage fees), has led to big gains for both companies. The only problem for the two upstart carriers may be holding on to the subscribers they are adding.

T-Mobile and Sprint are growing
T-Mobile (including its MetroPCS pre-paid brand) and Sprint (including its pre-paid Boost Mobile) beat out AT&T and Verizon during the quarter which ended March 31, 2015, according to research from Consumer Intelligence Research Partners. The research company explained its findings and methodology in a press release:

CIRP analyzes carrier gains and losses for customers who activated a mobile phone during the quarter. Using each carrier's percent share of these customers before their new activations as a base, we measure each carrier's relative performance. This analysis shows that among customers that activated a phone in the quarter, Sprint grew its customer base 25%, and T-Mobile grew its customer base 20%, relative to the number of customers that started the quarter as their respective customers.

Verizon and AT&T posted gains as well, they were just less significant.

Source: CIRP

"Sprint started the quarter with 14% of the phone activators in our quarterly survey, and ended the quarter with 17% of them," said Mike Levin, Partner and Co-Founder of CIRP. "This almost 25% increase in share of this quarter's phone activators suggests that their efforts to expand their customer base have started to take hold. Similarly, T-Mobile started the quarter with 19% of the phone activators in the sample, and ended with 22%, for an increase of almost 20%."

That's good news for the number three and four carriers as it shows that people are at least willing to give them a try. That's likely partly due to both companies marketing based on value and the fact that the network quality issues which AT&T and Verizon still trumpet as the reason people should pay more for their services have actually diminished over the past year, according to research from RootMetrics.

AT&T and Verizon customers are more loyal
While Sprint and T-Mobile have done a good job getting people to sample their service, they have not managed to hold onto them as well as AT&T and Verizon. As you can see on the chart below, the leading carriers are gaining fewer subscribers from other providers, but their existing users are less likely to leave. 

Source: CIRP

"AT&T and Verizon continue to have the most loyal customers," said Josh Lowitz, Partner and Co-Founder of CIRP. "Relative to their size neither attracts as high a percentage of new customers from other carriers as Sprint and T-Mobile do."

It's also worth noting that Sprint and T-Mobile both attract a significantly higher percentage of first-time phone buyers than their bigger rivals. That makes sense because new users, especially younger ones, are more likely to be price conscious.

You've got to hold on
Sprint and T-Mobile have completed part of the equation attracting larger numbers of people switching to their services than their more-established rivals. The second piece is holding onto those customers and the two companies need to work with exiting customers, find why they are leaving, and correct the problem.

Some churn is likely geographical as while both companies have made huge strides with their networks, the RootMetrics survey still shows more problem spots than Verizon and AT&T have.

If T-Mobile and Sprint can cut their subscribers lost to other carriers rate to somewhere around the 17% AT&T posted or even the 20% Verizon did, than the big two will truly have something to worry about. People are showing a willingness to move to the lower-cost carriers and if they can fix whatever is causing customers to not stay than there gains will grow exponentially.