Oftentimes while on a conference call, you will want to listen to the conversation but not necessarily contribute. During these long periods of silence, it is proper conference call etiquette to mute your phone's microphone so as not to distract the other members of the call with unimportant background noises from your phone. And not only is it polite, but it frees you up to complete other tasks without being heard. If you use your iPhone for conference calls, you can mute your microphone directly from the call screen.
Look at your phone's screen after you have joined the conference call. Locate the "Mute" option above the red "End Call" button. It is denoted by an icon of microphone with a slash through it.
Tap once on the "Mute" square with your finger. This will mute your iPhone's microphone, allowing you to hear the conversation while remaining silent to other members of the conference call.
Tap once on the "Mute" square when you wish to speak again.