I once sat next to a guy at work whose phone would induce this noise in my speaker about 10 seconds before his phone actually rang. So one day just before his phone rang I said 'Steve, your wife's phoning' and a few seconds later, she was. He was totally freaked out by this; he'd never noticed the buzzing sound. He had no idea how I had worked this out. His wife rang him all the time, statistically it was pretty likely to be her. :-)
Should not happen using a modern phone camping on a 3G network. 3G (WCDMA)is not based on TDMA and the uplink has been designed specifically to avoid creating audible interference.
But why does the interference stop once you answer? The author gives a couple of hypotheses in the comments, but might there be somebody here who knows for sure?
THATs what that is... And I thought I was crazy for hearing sounds before a phone rang (as a kid)
My Nexus has a ringtone (called radiation) that emulates this then segues into a little tune.