Scenario: When presented with an inbound call to your MS Teams App (desktop, mobile, MS Teams Handset) and you require to send that unanswered or rejected call to voicemail
Solution: Go into your MS Teams application → Select User Profile → Settings → Calls → Call Answering rules - Calls Ring Me (Checked) → If unanswered → Set to Voicemail
Microsoft Documentation: Manage your call settings in Teams