People with wireless AT&T contracts are happier with their customer service than those with any other carrier, according to the latest rankings from the J.D. Power research firm. Virgin Mobile earned the highest scores for customer service satisfaction among non-contract wireless service providers.

J.D. Power has issued the rankings twice a year for the past 13 years. It released its two latest surveys -- the U.S. Wireless Customer Care Full-Service Performance Study and the U.S. Wireless Customer Care Non-Contract Performance Study -- on Thursday.

The studies also found that customers increasingly prefer online chat as their service support channel of choice. A growing percentage of customers -- 18 percent this year, up from 9 percent in 2011 -- report using online chat, which is the channel showing the highest rates of first-contact resolution of customer problems.

Chat 'Efficient and Immediate'

"For wireless customers, online chat is an efficient and immediate experience for problem resolution," said Kirk Parsons, senior director of the telecom services practice at J.D. Power. "This contributes to higher levels of satisfaction, especially pertaining to service issues or questions such as billing or service and device-related issues associated with upgrades."

Parsons added, "As carriers release new products and services, such automated systems as online chat must continue to evolve to address harder-to-answer questions related to technology support."

Other forms of support examined by the surveys included walk-in (retail store) support, automated phone services and phone-based support from customer service reps.

J.D. Power's full-service rankings are based on responses given in the second half of 2014 by nearly 6,500 wireless customers, while the non-contract study surveyed more than 2,600 customers. Both surveys questioned people who had contacted their wireless carriers' customer care departments over the past six months.

Problems, Questions, Account Changes

On a 1,000-point scale, AT&T earned 786 points for full-service wireless customer care, compared to the average score of 773. T-Mobile came in second with 777 points, while Verizon Wireless earned 771 points and Sprint placed last with 746 points.

"AT&T performs particularly well in the walk-in channel and ranks above the full-service average in four of the five service channels," according to J.D. Power.

Among non-contract wireless customers, Virgin Mobile took the top spot for customer service with 775 points, performing especially well in telephone and online support. Next came Boost Mobile with 747 points, MetroPCS with 737 points, and Cricket with 730 points. The lower-end performers earning less than the average score of 726 included TracFone (692 points), Straight Talk (691 points) and Net10 (674 points).

The surveys looked at three types of customer service issues: problems, questions and account changes. Depending on the issue, customers preferred different channels for support, J.D. Power found.

Full-service customers, for example, tend to go online more often for questions (53 percent) rather than for problems (25 percent) or account changes (15 percent). They also use the phone for questions (40 percent), problems (38 percent) and account changes (18 percent). Problems were the top reasons (43 percent) for walk-in retail service, followed by questions (36 percent) and account changes (15 percent).