How to mute while hands-free on headset?

I frequently have to join multi-point conference calls whilst on the move, either in my car or walking when I don't have the phone handset accessible. I have to be able to quickly switch to unmute when my input is needed, but then need to be back on mute again so that noise from wherever I am does not interrupt the conference call. On my Blackberry this was simply by pressing a button on the wired headset control, but when I tried this on the iPhone it hung up the call - very embarrassing.

I can't for the life of me work out how to do this on the iPhone standard wired headset, the only way I can find to do it is on the handset - but that is not always possible, if I have my hands full or if I'm driving and the phone is in my pocket.

I'm sure it must be a certain button combination, but I can't figure it out! Help!