Guests being removed from lobby due to possible inactivity

Hi,

We hold large meetings, during these meetings, we occasionally have guests that'll dial to join the meetings. They have scheduled slots to which they allowed to attend. On multiple occasions, we ensure the guest who is joining the meeting will sit in the lobby until they're approved. 

Sometimes they can be sat waiting in the lobby for 15+ minutes. I've found that these guests will suddenly drop out. Now I can appreciate this could be down their own phone signal but for this to happen to multiple people in different meetings would suggest there could be a possible inactivity time frame that may remove that guest from the lobby due to inactivity. 

Has anyone else had any similar issues that sound familiar to what I've explained in the above?