Your Microsoft SBC connects your Microsoft Teams users to your Direct Routing provider. Without an SBC, there would be no way to send media and voice between devices via the PSTN. In addition to terminating SIP trunks, your SBC acts as your voice firewall. Whether you choose to host your SBC or not, Microsoft Teams will send calls to the SBC, and then to the PSTN or emergency services provider with IP-based SIP trunks.
Learn more about hosting your Teams SBC.