I have this vision of someone answering a phone version of it and holding it up to their ear. "HI.. YEAH I'M ON THE TRAIN..."
If you were technically inclined, I guess you could wire a headphone-equivalent resistor to a 3.5mm jack and plug it in when you want to "mute" it. Or await a jailbreak that lets you mute it fairly easily from sbsettings or something.