Additional meeting and call-related features and issues within Microsoft Teams for business
Teams currently allows you to choose from the available built-in ringtones under:
Settings → Calls → Call handling and forwarding → Choose a ringtone
However, Microsoft does not currently document a supported option to upload your own .mp3, .wav, or .ogg file as an incoming-call ringtone.
I had confused the custom notification sound functionality under Notifications and activity with the separate call ringtone setting.
So, for the original question, there is currently no supported drag-and-drop/upload method for using your own audio file as a Teams call ringtone. I don't recommend modifying Teams application folders as a workaround.
Apologies for the incorrect information in my original response.
Reference:
Manage call settings in Microsoft Teams
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