So when FreePBX calls Teams then Teams tries to transfer the call to another Teams user… the call fails
I hope I made sense… Here is my log from SBC Mediant1000
Could you post Asterisk logs (/var/log/asterisk/full) and SIP capture information from FreePBX’s side?
That SBC log is… something for the SBC vendor to deal with! 
But one thing I can spot here is that the SBC is using a REFER method to perform a blind transfer.
Aug 3 13:11:28 local0.notice [S=44548757] [SID=ae35fe:56:832217] REFER sip:[email protected]:5067;transport=tls SIP/2.0
FROM: <sip:[email protected]>;tag=d2b6514476e74386b407e5eff2004a45
TO: <sip:[email protected]>;tag=1c577941657
I don’t know that FreePBX can accept this without special handling. (res_pjsip Remote Attended Transfers - Asterisk Project - Asterisk Project Wiki)
You might have to create some custom dialplan to define the external_replaces extension, per the linked guide.
Or figure out how to set the SBC to supervise the transfer and not use the REFER method.
edit: Never mind; I’m probably wrong. That REFER is not on the SBC/FreePBX call leg. If you can get the asterisk logs & SIP, that would help clarify what’s going on from FreePBX’s perspective.
I didn’t know any other way to send you this file, edge kept not responding to me