Microsoft Teams does not naturally send a REFER message to your PBX when a call is transferred or parked. This causes broken SIP flows or incomplete information for normal processing. You must opt in to that functionality on each direct route to use REFER.

Benefits of enabling REFER: 

• Call park and transferring will work as is typical.

Drawbacks of enabling REFER: 

• Attended transfers will not work unless the PBX hairpins the transferred call back to Teams where the media is anchored. 
• The only supported codec is G.711.