Hi Pe11ant,
Could users join the federated online meeting with SFB Web App?
Did users get any errors when they join the SFB meeting?
According to your description, this issue occurred after you configured the hybrid environment. For this issue, I suggest you could check the following settings in your environment:
1. Check the Ports and Firewall settings in your environment, please make sure you have enabled ports TCP/UDP 50000-59999, and firewall rules did not block the traffic.
2. Check the federation settings in the SFB control panel, make sure you have enabled federation access in the policies.
3. Check the settings in the SIP Federated Providers, if possible, you could try to delete then recreate it.

Best Regards,
Evan Jiang
Please remember to
mark the replies as answers if they helped. If you have feedback for TechNet Subscriber Support, contact
tnsf@microsoft.com.
Click
here to learn more. Visit the dedicated
forum to share, explore and talk to experts about Microsoft Teams.