[jarek]

It is the same kind of chatter you might be able to hear in speakers when the phone is nearby? That's a standard GSM thing -- GSM uses specific RF frequencies which induce the noise. ("GSM chatter" is a reasonably common name for this.)

[nitrogen]

This is one of the reasons I prefer CDMA. It is really, really annoying to be in a public address setting (i.e. a church, auditorium, stadium) and hear peoples' phones in the sound system. Yes, it is a sign of a miswired installation, but in my experience it only happens with GSM phones.