Microsoft Teams Direct Routing (Service Provider Model)

Direct Routing supports configuring one Session Border Controller (SBC) to serve multiple tenants.
A carrier:

Deploys and manages an SBC in their datacenter (customers don't need to implement an SBC, and they receive telephony services from the carrier in the Teams client).
Interconnects the SBC to multiple tenants.
Provides Public Switched Telephone Network (PSTN) services to customers.
Manages call quality end to end.
Charges separately for PSTN services.