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OSSmosis 5 -> Telephony Setup -> Music on Hold (Reference: OSSmosis: Music on Hold)

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III. Microsoft Call Queues: Configured in Teams Admin Portal

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MS Teams Music on Hold is NOT SUPPORTED in the follow use case: 

IV. Teams to Teams Calls: When on a Teams to Teams call, there is SILENCE and no HOLD music.

This is currently noted as a future enhancement to Microsoft Teams ***UPVOTE MS Teams Voice Request HERE***

Workaround: Use Call Park and Retrieve feature to trigger the HOLD MUSIC