Mute Upon Entry is available on the WebEx scheduling page for Training Center and Event Center. Using either of those WebEx services you can schedule a session whereby all attendees will enter on mute:
TC & EC Option During Scheduling:
When using Meeting Center, this option is not available at time of scheduling, but can be activated by the Host once s/he has started the meeting by pulling down the Participant panel and selecting Mute on Entry:
MC Option In Your Live Session
Hope this helps.
Susan
WebEx Onboarding and End-User Training Services