Each leader account is assigned to their own ‘meeting room’, attendees can enter the room when they use an active meeting ID (an ID is like a key to open the door to the room)
If a meeting is in-progress (people in the room) and attendees from the next meeting use their meeting ID (key to the room) they will join the in-progress call.
To avoid this, a leader can lock the meeting room which prevents attendees from the next call joining the current meeting,
How to lock a meeting room
1. Click the padlock icon on the Live Call page, click the button again to unlock the meeting
2. Enter *2 on your telephone keypad to lock the meeting, enter the command again to unlock.