In the RMX, this functionality has been moved to the DTMF codes in the conference IVR services. It is now a chairperson command called "Mute All Except Me." The default value for this code is *5, while #5 is the command to unmute.
The conference Chairperson can be defined manually, by a bridge operator. Or a particpant can sign in as a chairperson by using the DTMF code assigned to that function. By defaul, it is *76. Participants using the DTMF code to make themselves the chair will need to enter the chairperson password for that conference.
Hope this helps,