Voice for Teams, Direct Routing Overview

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