It's not a new idea. Short audio "logos" have been used to identify radio shows, TV networks, long distance phone companies (during the long distance wars in the US in the 1990s) and, most recently, podcasts. But mobile operators have missed this opportunity, at least so far.
Now SK Telecom has introduced the idea in Korea and a number of other mobile operators are scrambling to leverage the idea. In Korea it's called T-Ring and it's both a branding exercise and a key part of a special pricing bundle.
When you call an SKT subscriber, the first thing you hear is a 1.5 second sound clip — the SKT audio logo. Then you heard the called subscriber's ringback tone (assuming they are among the 55% of Korean subscribers who pay for ringback tone service) or you hear normal ringing signal, until the subscriber answers.
Everyone hears the T-Ring sound, letting them know they are calling an SKT subscriber. Even better, SKT has introduced a pricing bundle where subscribers get 50% off on calls to other SKT numbers. As a Korean friend puts it,
Of course it is nice to hear the sound before the connection is made because your brain is naturally set to read "Oh 50% discount."
This is also good for NMS Communications, as the equipment is similar to that used for ringback tones and we already provide components and platforms to most of the major ringback tone suppliers worldwide. :-)