Tonya Carter used to buy cell phone ringtones from her mobile service provider, Verizon Wireless. However, lately she's found a way to bypass her carrier and download them straight from the Web. Carter, a 33-year-old Houston resident, pays about a month for a service called "Thumbplay," which gives her access not only to ringtones but to a gamut of games and graphics. A growing number of content-hungry consumers are circumventing carriers, depriving the likes of Verizon Wireless, AT&T and Sprint Nextel of a key source of revenue.