Ringback tones break out in song.

Choosing a ringtone for your wireless phone is old hat. But choosing the ringtone people calling you hear--instead of that boring "brrrrr, brrrrr"--now that's innovative. Feeling funky? Let 'em hear the latest pop hit. In a mellow mood? Perhaps a running river sound is more appropriate.

Users can customize different songs for different callers, so a subscriber's girl-friend might hear a love song while waiting to connect, a schoolmate might hear the latest rap song, and a parent a Frank Sinatra hit. Songs are also customizable for different times of day. Unlike ringtones, which are downloaded and stored in a cell phone, ringback tones are stored in a server at a central telecom switch, so users do not need to upgrade handsets.

Developed and introduced last year by South Korea's SK Telecom, personalized ringback tones, also known as color ringtones, have proved to be a huge hit throughout the Asian market. In Asia, the color ringtones are priced between $US0.75 and US$1.50 per month. With millions of Koreans adding this feature to their phones, the service has provided an unexpected boon to the bottom line; according to the Yankee Group, South Korean wireless carriers collected a total of $91.4 million in ringback revenue last year. By the end of this year, 9.5 million South Koreans will use ringbacks, spending what some industry watchers estimate will top $100 million or more.

U.S. and European carriers are also investigating the possibilities. In the U.S. market, for example, 15.7 percent of cell phone subscribers either use ringtones or would like to have them, while 65 percent rate ringtone functionality as a primary driver for upgrading a handset, according to research firm Telephia. Analysts say that type of success could easily be duplicated with ringback tones.