Outbound calls from my SBC into Teams (Polycom -> SBC -> Teams) always result in a 400 BAD REQUEST.

Example invite below:

https://pastebin.com/F1G1Ce59

I've taken care to make sure numbers are all E.164 format in From/To/Contact. I've also taken care to make sure that FQDN is used in Contact and Record-Route header.

I've tried many different variations and have followed the SIP information here:

https://learn.microsoft.com/en-us/microsoftteams/direct-routing-protocols-sip

I've also tried several different clients (Bria, Polycom CCX 600, Grandstream, etc) to see if maybe it was something in the SDP or otherwise causing an issue.

SIP Transport is TLS, RTP is SRTP

I might also add that OPTION pings are active and Direct Routing Endpoint is active, so this is successful. I also am able to receive calls FROM teams to my IP phone(s) without issue. It is only when I try and call INTO teams (INVITE -> Microsoft Teams) that I always receive a 400 BAD REQUEST to my INVITE.

Any help is appreciated, thank you!

- Brandon