No, it is not necessary for all of your participants to join via video in order to access your conference call. If some of your participants don't have a webcam or don't have access to the internet, they can still join your conference call.
Those joining your conference from their computer can access the call in exactly the same way. The only difference will be that there will be no video available for that participant. See the FAQ 'How Do Participants Access My Video Conference?' for more detailed information.
Joining By Phone
Callers using their phone can call in with your regular Dial-In Number and Access Code. Part of their number (excluding the last 4 digits) will be visible on their tile via your dashboard. Full numbers will be displayed to moderators with paid subscriptions.