Microsoft
Microsoft Office 365 Enterprise service with either E5 licenses, or with E1-E4 licenses and the Phone Service add-on.

8×8
An 8×8 X Series account.
One 8×8 X series License (X1-X8) must be assigned to each user of Microsoft Teams.
MS Teams licenses must be applied to your account in the 8×8 Admin Console, with one license assigned to each user of Microsoft Teams.
Implementation services are required for all 8×8 Voice for Microsoft Teams deployments.

Configuration
Setting up the service must be performed by one or more administrators who have access to admin functions of both Microsoft Teams and the 8×8 Admin Console.