Weird thing, but since writing the reply above this have began happen to me a lot.
Same MO every time: while listening to music or a podcast, I begin a call using Voice Control (no Siri). A second or so after the call is accepted, the music/podcast begin to play in the background of the conversation.
It happened enough times that I'm sure there are no involuntary button presses involved. It also happened using three different headsets (original white ones, Creative and Klipsch) so I think it's safe to rule them out.
Anyone else experiencing this?
I'm using a tired old 3GS, never jailbroken, on iOS 6.0.