Verizon has come out on top in J.D. Power and Associates’ latest wireless-call quality study, garnering the highest rating in four out of six U.S. regions studied.

The study, which is conducted semiannually and is based on responses from more than 25,000 wireless users, ranks carriers based on customer-reported problems, such as dropped calls, static and voice distortion. According to J.D. Power’s call-quality index rankings, Verizon has the highest call quality in the Northeast, Mid-Atlantic, Southeast and West regions. U.S. Cellular is tops in the North Central region, while AT&T has the highest ranking in the Southwest. Alltel is tied with Verizon for No. 1 in the Southeast.

Although Verizon ranked well in many U.S. regions, the company still has room for improvement, because it tied for last along with Alltel and Sprint Nextel in the Southwest, trailing behind AT&T and T-Mobile.

Among the bigger gainers in the survey was Sprint Nextel, which saw its call-quality index rating in the Southeast shoot up to 105, up significantly from the region-low 93 score it earned in the September 2006 version of the study. The company also improved its rating by 5 points in the West region, scoring a total of 107. T-Mobile had both gains and losses from last year’s study, boosting its score in the Mid-Atlantic region to 109 from 101 last year, while watching its score drop from 107 to 102 in the West.

Overall, the study found that the quality of wireless calls has been improving steadily for the past three years. In particular, it noted that the number of customer-reported call problems dropped from 21 per 100 calls over the same reporting period last year to 15 per 100 calls this year. It also found that callers who use such hands-free devices as wired headsets experience about 20% more problems with their calls than customers who don’t use hands-free devices.