In todays age where you get to hear artifical voices expressing their concern that you cannot talk to the required person because either they are out of reach, phone is busy or switched off, and in some cases getting the chance to leave a message, all in variety of languages some of which you dont know and dont care to know, a simple engaged tone leaps out of the blue at you and heads straight to the jugular.
I rechecked the number on my phone.
Afterall the beauty and simplicity of a single repetitive note can express in one beep what a full sentence may not. And save time of the caller.
Same can be said of the "caller tunes". One can never figure out whether that was one ring note or two, or 10.