New figures indicate Verizon maintains the best mobile network quality but it is dropping. Reuters

The latest network performance numbers are putting large wireless carriers Verizon and AT&T (NYSE: T) on notice: bigger is not always better.

In wireless phone service, No. 1 and No. 2 providers Verizon and AT&T have traditionally dominated measures of network performance over smaller rivals Sprint and T-Mobile. But that's changing. Analytics firm RootMetrics says in its latest ‘State of the Mobile Union’ report that while Verizon Communications (NYSE: VZ) remains the overall winner in terms of service quality, it slipped in performance from last year and No. 4 carrier T-Mobile is gaining.

RootMetrics State of the Mobile Union results. RootMetrics

RootMetrics network performance reports, which compiles mobile data on network reliability, speed, data performance, voice performance, and text performance from all around the nation rates Verzion at 81.6 while AT&T comes in second at 79.5. However, both networks have seen significant drops on their performance rating over the past year.

Meanwhile, T-Mobile US Inc. (NYSE:TMUS) has not only improved its score from 64.3 to 71.5, it has also surpassed Sprint Corp. (NYSE: S) in terms of service quality. Though Sprint comes in last, the network has improved its performance report score from 68.2 to 69.6. Sprint and T-Mobile are respectively the third and fourth largest networks in the U.S.

T-Mobile has spent the last 18-months improving its network and building its subscriber base, which easily accounts for its sharp increase in quality, according to RootMetrics. While T-Mobile trails closely behind Sprint in terms of subscribers, the T-Mobile plans to surpass Sprint by the end of the year. Recon Analytics analyst Roger Entner believes T-Mobile can beat Sprint by the end of the quarter.

But Sprint does not plan to go down without a fight. With a new CEO in place, Sprint is gearing up to gain customers and improve its network with new competitive rates for its individual and family plans. Sprint CEO Marcelo Claure recently told employees at a company-wide town hall meeting that the network now has three priorities: reduce prices, improve the network, and decrease operational costs. With Sprint’s moves, a price war is now brewing, which could affect the outcomes of States of the Mobile Union in the future. RootMetrics figures account for the first half of 2014.