Leap Wireless International, the parent company of prepaid carrier Cricket Communications, posted mixed fourth-quarter results as losses worsened but churn improved and the company grew its subscriber base.

The company’s losses ballooned to $249.4 million in the fourth quarter 2010 from $61.9 million during the same period in 2009 after the cost of a $1.1 billion debt buyback, the formation of a joint venture in south Texas and charges related to the company’s buyout of the remaining shares in Denali Spectrum offset a 13 percent rise in sales, which hit $708 million.

However, the company added 430,000 subscribers during the fourth quarter with 107,000 net adds and 323,000 subscribers gained through the acquisition of Pocket Communications in south Texas. The company ended the year with about 5.52 million customers, an 11 percent increase over 2009.

Churn improved to 4 percent, and the turnover rate for Leap’s voice customers dropped to its lowest level in nearly a decade at 3.6 percent. ARPU dipped slightly over last year to $38.14 as former Pocket Communications customers, who are on lower rate plans, became part of Leap’s customer base.

Leap has been working to bring its offerings up to par with competitors like MetroPCS. The company introduced its third smartphone, the Android-based Huawei Ascend, during the fourth quarter; launched nationwide 3G roaming; and recently began offering its Muve Music service to its customers. The company’s purchase of Denali Spectrum gave it control over Denali’s Chicago and southern Wisconsin markets, and Leap formed a new joint venture in south Texas through its purchase of Pocket Communications, bringing its penetration rate in the region to the high teens.

Doug Hutcheson, Leap’s president and CEO, said the company sold more than 600,000 3G devices during the fourth quarter. About 10 percent of Leap’s customers now use smartphones, and Hutcheson said device upgrade activity more than tripled over last year and two-thirds of Leap’s subscriber base is on an all-inclusive rate plans.

Leap customers who upgrade to a new device and migrate to the company’s new service plans are about 60 percent less likely to deactivate service within five months than those who do not, Hutcheson said. The company plans to introduce seven new devices in the first half of this year and has a “service plan refresh” slated for the second quarter.

For the full year 2010, Leap’s losses more than doubled to $785 million despite an 8.7 percent increase in sales, which hit $2.7 billion.