How do you turn off beeping sound when you put a call on hold

I spend a lot of time on conference calls and get calls coming in on the other line which I want to switch over to. The iPhone's hold feature is great for just popping over quickly to the other call and then coming back to the first call, the problem is that there is a beeping sound to the audience on the fist call while they are on hold. Is there anyway to turn this beeping feature off, I don't want to disrupt the conference call while I flip over to the second call? Does anyone know if you are on mute, does it still beep for the party on the first call?