Direct Routing uses the pstnhub proxies on 5061, a certified SBC, and a verified domain. Contact must be an FQDN.
Direct Routing signaling uses sip.pstnhub.microsoft.com, sip2.pstnhub.microsoft.com, and sip3.pstnhub.microsoft.com on port 5061. TLS 1.2 or later is required, and the SBC is a certified SBC.
The synthetic INVITE targets sip.pstnhub.microsoft.com:5061 from sbc1.example.com. The real routing among the three proxies is in the plan and protocol articles.
The Contact must be an FQDN. If Contact is an IP address the proxy returns 403. The domain on the trunk must be a verified tenant domain, not a name ending in onmicrosoft.com.
The protocols article says the SIP proxy does not support RFC 5923 connection aliasing. Microsoft says wildcard names in the certificate CN or SAN are supported for Direct Routing.
Room: Teams Direct Routing signaling
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