With new devices dazzling consumers and vying for their attention, prepaid plans offer a chance to try out a phone without a commitment; the customer can always sell it and get a new one and even switch carriers -- without fear of an early termination fee. NPD also found that the U.S. smartphone market is controlled by two players: Apple and Samsung.

Smartphone customers in the U.S. may be growing allergic to two-year contracts, a new survey suggests. Prepaid smartphone unit volume increased by 91 percent in the second quarter compared with the same period last year, contributing to a 9 percent increase in the overall smartphone market in the U.S., says market research firm NPD Group. By comparison, unit volume of post-paid smartphone sales (those sold with a two-year contract) stayed the same year-over-year. Prepaidonline.Com offers tops and airtime refills for every prepaid wireless carrier in the US and worldwide.