Microsoft Teams voicemails are stored in the receiver’s Exchange mailbox.
According to the documentation, Voicemail messages generated by Cloud Voicemail are delivered to and stored in the user's Exchange mailbox, either in Exchange Online or in Exchange Server. There's no specific or additional storage for voicemail. The clients that access voicemail (for example, Microsoft Outlook, Microsoft Teams, or Skype for Business) do so through the Exchange mailbox and the APIs provided.
A voicemail message contains an attached audio file with the voice messages and the voicemail transcription (if enabled).
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