With folks such as the Black Eyed Peas, D4L, and even "Crazy Frog" leading the way, ringtones are well established as big business and show no sign of letting up.

Revenues from the sonic snippets that mobile phone users can't seem to get enough of will surpass $600 million in 2006 in the US, according to BMI, the performing rights organization that represents more than 6.5 million musical works on behalf of songwriters, composers, and music publishers.

That mark would top 2005's revenues of approximately $500 million and would be a massive spike from $245 million in 2004 and $68 million in 2003, respectively.

"We see the market maturing in 2006 with growth fueled by an increase in multimedia handsets and more aggressive marketing by wireless carriers," BMI's Richard Conlon said in a statement. "We believe that the market will grow by 20 percent for the ringtone sector alone in 2006."

Conlon noted that 2006 will be a make-or-break year for the "ringback," in which the usual ringing tone heard by a caller is replaced by a downloaded musical snippet.

BMI tracks census sales data on more than 325 outlets for the sale of mobile entertainment in the US. Its projections are based on ringtone transactions over the past seven quarters.