J.D. Power and Associates says as the number of smartphone users grows, wireless carrier stores are providing their smartphone customers with a more satisfying retail experience than users of more traditional handset models.

J.D. Power’s semi-annual study finds that satisfaction with the retail experience among smartphone owners averages 15 index points higher (on a 1,000-point scale) than that of traditional wireless handset owners, even though smartphones typically require additional service plan options and have more complex features.

On average, satisfaction with the retail experience among owners of smartphones averages 722, while among owners of traditional models have a satisfaction score of 707.

Smartphone owners are more satisfied than traditional handset owners in all factors, particularly with regard to price and promotion even though smartphone owners pay more for their devices and spending $23 more per month on average for their service.

AT&T has placed considerable focus on its smartphone line-up, but in the index, it came in below T-Mobile USA, which scored 729. T-Mobile was followed by Alltel, which scored 721. Verizon Wireless was third at 714, followed by Sprint Nextel with 710. AT&T indexed at 700.

The research firm says that overall satisfaction with the retail sales process decreases dramatically when certain key activities are not performed by sales staff. For instance, satisfaction is 139 index points lower when a salesperson fails to offer to explain how to operate a phone, which is particularly important among smartphone users and customers who purchase feature-rich handsets.

Smartphone owners who visit retail stores report spending more than four additional minutes in the store during their most recent visit compared with owners of traditional handsets.