LONDON - When Alexander Amosu's cell phone rings, he really listens - to the ring itself.

Hardly anyone is more attuned to that sound's possibilities than Amosu, 28, a pioneer in the multibillion-dollar global ring tone industry.

If Amosu's phone makes the sound of a siren, he knows it's his mother calling. "I know I must be in trouble," he explained.

If it's the hip-hop tune "21 Questions" by 50 Cent, he knows it's someone from work. If it's the song "Pump It Up," he knows it's a friend.

So what happens if it's his wife calling? Then it sounds just like a typical phone.

"I used to have the phone play Shaggy's 'It Wasn't Me,' but I guess the song is about cheating, and so she made me change it and change it fast," he said, laughing.

When it comes to ring tones, Amosu has probably tried them all - and that's hardly a surprise, since he created many of them himself.

In 2000, the personable entrepreneur became one of the first to tap into the ring tone trend when he founded R 'n' B Ringtones, which enjoyed a turnover of 1.2 million pounds (about $2 million) in its first year.

Two years later, the man dubbed "Lord of the Ring Tones" by the British press received the Black Enterprise Award for "Young Entrepreneur of the Year," handed out annually by an online publishing firm, the Colourful Network, to celebrate African and Caribbean business people in Britain.

Even though Amosu sold 80 percent of his company last year to an undisclosed buyer, he still provides it with nearly 20,000 ring tones of all kinds.

"I must have been onto something, because today there are 257 ring tone companies in the U.K.," he said.

The self-made millionaire was one of the first to find fortune by serving the growing wave of those eager to personalize their mobiles.

Consumers around the world are spending billions of dollars a year on the downloading of ring tones. Pop musicians have even started releasing ring tone versions of their songs when they release a new CD.

Indeed, according to analysts, mobile ring tones outsold CD singles for the first time in both Britain and the United States last year. Woolworth's introduced a top 10 ring tones chart in its stores here late last year alongside their charts of top 10 CD singles.

Besides music, consumers today also have the ability to download a smorgasbord of real-life noises, from a baby's cry to the unsettling sound of a car crash.

The research company IDC estimates that Americans shelled out more than $57 million on ring tones in 2003, compared with only $16 million in 2002. Ring tone sales in Japan and Europe totaled about $5 billion in 2003.

Vendors typically charge about 99 cents for monophonic tones to about $2.50 for higher-quality polyphonic versions.

Some analysts believe ring tone sales in Europe have peaked.

Amosu now has his sights set on a new phenomenon: video messaging.

Amosu recently launched www.mobsvideo.com to sell 30-second video clips for the cell phone.