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End user account must be homed in Office 365. You can’t use Teams Voice for the same end user account that is homed on-premises SfB service. (Microsoft reference)
End user must have the following licences/usages assigned. (Microsoft reference)
Microsoft Teams Phone (aka Phone System). Add-on license for E1/E3, included in E5.
Microsoft Teams. Included in E1/E3/E5
*Microsoft Audio Conferencing. Add-on license for E1/E3, included in E5.
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