Requirements
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.
*Not mandatory for end user voice service, but the Audio Conferencing license is required for adding external phone participants to scheduled Teams/SfBO online meetings, either by dialing out to them for the meeting or by providing the dial-in number. Conferencing dial-in phone numbers from Microsoft.
Solution architecture