We have DTU16D phones.. how do I put speakerphone on mute?! I press FEATURE 1, but the person I am calling can still here me, just not as loud. Is there a way to shut it off completely for the time being?!

We have a conference call later today and they want us to just listen. There will be many people on. They will be showing us a Powerpoint presentation on the web and we'll type our questions through a web application.