Direct Routing lets you connect a supported, customer-provided Session Border Controller (SBC) to the Microsoft Phone System. With this capability you can configure on-premises PSTN (bring your own sip trunk, BYOT) connectivity with your Microsoft Teams client.
There are a couple of certified SBC vendors you can use to setup Direct Routing.
Audiocodes Ribbon AnyNode Thinktel What do you need? In order to setup Direct routing you will need to have some prerequisites: