With Moderator features (available on the Plus and Pro plans), WHYPAY? conference organisers can can specify when editing a conference room that the conference only starts once a Moderator has joined (more on Moderators here). 

Checking the Moderator required box when editing/scheduling a conference will mean that anyone dialling in using the standard 4-digit PIN will be placed into a 'waiting' state (they'll hear your chosen background music) until the conference has started.

Once a Moderator has dialled in (using the 5-digit Moderator PIN), the conference will start and all participants will be connected to the conference call.

Note that when all Moderators have hung up, all Guests who have dialled in using the standard 4-digit PIN will automatically leave the conference too.