What do people really do with their wireless phones?  Not always what the Tier 1 carriers, handset companies and mobile content providers would have you think. According to NPD Group, a massive 45% of U.S. mobile phone users just make telephone calls.

“Only 20% of mobile phone users prefer to use their phones as an all-in-one multimedia device for music, videos, Web surfing, and other activities beyond making phone calls,” the research firm said today.

Customers of T-Mobile are mostly likely to use advanced features, followed by customers of Sprint, and trailed by customers of AT&T. “Verizon Wireless customers are least likely to embrace their phone as an all-in-one multimedia device,” NPD said, despite that company often being cited for having the industry’s best customer service and widest coverage.

Verizon, as of Friday this week, will officially be the nation’s largest mobile phone company.

“Carriers and other handset retailers have an opportunity to educate customers as to the capabilities of their handsets in the wake of slower overall handset sales,” analyst Ross Rubin explained.

Of the customers who do want more features than just simple telephony, the most requested option is a cameraphone, followed by text messaging, Rubin said in another recent study.