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Ringbacks that play alternative audio instead of the standard ring to callers while they are dialing a number are starting to cause a commotion with U.S. mobile users.

The number of U.S. ringback subscribers tripled last year, according to M:Metrics. A recent J.D. Power and Associates study also found that 9 percent of U.S. mobile consumers used ringbacks. IDC estimates that 40 million global users will subscribe to ringback tones by 2010.

"In North America, 10.8 million people are using ringbacks today," said John Orlando, vice president of marketing at NMS Communications LiveWire Mobile. "Places like Spain are north of 10% penetration, India is near 40% and markets like Korea have stabilized around 60%."

In a likely unpopular move ringbacks could include advertisements. Subscribers friends would be forced to listen to the ads. The subscriber would receive discounts on mobile content or free minutes, and the mobile provider would collect the ad revenue.