Hi there,
Thanks for posting. The system is designed to allow calls to ring through if you're just in a meeting. Details of all the presence states and how they work are here -
https://docs.microsoft.com/en-us/microsoftteams/presence-admins You should be able to stop this from happening by shifting into Do Not Disturb (DND) during the meeting that should stops calls from ringing through. If you do see a call come through while you are presenting or have set yourself to DND that's a bug and you should escalate through your support channel.