Is there any way to prevent Teams communication to our on-prem Skype For Business users (we're in Island mode) from federated partners in TeamsOnly mode (who are upgrading from Skype For Business) without disabling federation to those partners which would block SFB - SFB and SFBO - SFB communications ? We can't tell our partners to switch to Islands mode where the Teams - SFB on prem communication is prevented from working. Third party compliance solutions have not been enhanced to capture these Teams - SFB communications yet.
If you have a compliance solution for that SFB federation, it should catch it. What am I missing?
In the future we intend to introduce a native Teams to Teams federation. At that point the path would no longer go through SFB federation. That said you should be able to block that at the Teams federation parametering level.
Other option is to not use Islands and go to the interop modes instead.