In the beginning of conference microphones cannot be unmuted.
It's necessary to access SMC WEB-interface, System->Settings and change parameter "Mute microphones upon starting conference" to "Unmute all" to make microphones unmuted when conference is starting. But microphones are still muted after this parameter has been applied.
The issue is that pop-up menu values and real parameter settings are mismatched.
To make microphones UNMUTED you should choose "MUTE ALL". To make microphones MUTED - "UNMUTE ALL".
SMC version: SMC2.0 V100R003C10SPC900