I am getting an error from CreateMeeting API that join before host is not available for the meeting type. I assume this is a site configuration that only a site admin can view. Can you verify whether there is a meetingType value in the sandbox that supports joinBeforeHost?
"JoinTeleconfBeforeHost is not supported for specified telephony type"
I discovered it was because I removed the telephony element. I had removed it because the examples show a ahrdocded phone number, which for most cases I would think you wouldn't want to hardcode the phonenumber+ext since that is within the domain of Cisco's control.