Generate a constant noise while in the cab, however you like, but make sure that it is not controlled by the vehicle's speakers otherwise this will not work. Now with that noise running constantly, perform your bluetooth phone call/satnav function simultaneously. If, when you are receiving driving instructions the other person hears a break in the constant noise, the satellite navigation probably has turned off the mic function for the duration of its instruction and then turned it back on after it is done. (since it does the same thing to the music this is probably the case) the person on the other end probably just doesn't normally notice the break in sound because the ambient noises aren't that loud and normally people aren't talking while they are receiving driving instructions.
If however, they do not hear a break in the noise, then i suspect magic and or Kid's explanation