Call flow — Teams client, SIP proxy, SBC, carrier
Source: Plan Direct Routing, accessed 14 October 2026.
1. Client marks audio when QoS is enabled.
2. Teams sends SIP/TLS to the PSTN hubs on TCP 5061.
3. SBC presents a public certificate. The paired FQDN is in the name.
4. OPTIONS. Repeated misses pull the trunk out of the route.
5. INVITE carries the normalized E.164 number.
6. 100 Trying, 180 Ringing, 200 OK, ACK.
7. Media uses the UDP ranges, or a bypass path you designed.
8. BYE tears the dialog down. Recording needs its own policy grant.
Emergency: resolve a dispatchable location before queue music.
Do not place a real emergency call to test this sheet.