As the cash cow that was the ringtones market slowly heads to pasture, the music industry is turning to a new mobile stud -- ringback tones.

Ringbacks represent the only area of significant growth for mobile music-related applications in the last year. The number of mobile subscribers who reported purchasing a ringback tone increased 69% from February 2007 to February 2008, according to data from M:Metrics. By comparison, neither ringtones nor wallpaper images could keep pace with the overall market growth, increasing by only 4.3% and 6.2%, respectively, in the same time frame.

"The growth of ringbacks is among the fastest trajectory for growth of any services we're tracking," M:Metrics analyst Mark Donovan says.

For a music industry that has long relied on ringtones to pad its digital revenue pie, this has several implications, both financial and promotional.

Click here for the full story including details on this year's projected ringback sales, the leading operator's secret to success, the key to expanding the format and more.