Silent ringtones

The idea of a 'musical' road isn't new, but the comments to this post contained this gem I hadn't known:

Actually in Japan they sell ring tones for silent mode. These are designed so that if you put your phone on silent+vibrate and place it on a table or some other resonant surface then you can get the table to hum a tune.

Update

I found a truly silent ringtone. Not a bad idea. But I can't find any reference to these supposed Japanse vibrating ringtones.