IPhone 6s Plus, how do you mute a call while on call? (not how to mute ringer)?

I have a new iPhone 6s Plus and need to listen in on conference calls without other people on the call hearing me type in the background or the kids, so how do I Mute the sounds on my end so I can listen in without disturbing the others on the call? My current phone keypad on the iPhone 6s Plus only shows 1-9,0 *, and # buttons.

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