We are having an issue where we have an on-prem SfB at a hosting provider to get dial tone and naturally Teams in O365.
To get SfB to work outside of O365 we had to deactivate the Skype-license on all users and disable external federation. If not, other O365 SfB users would search for the users in our Tennant.
Now more and more users are starting to use Teams they want to be able to contact external users on SfB and Teams. This is where the problem starts......
Now with the combined control panel in O365 for SfB and Teams I can activate or deactivate the External fed on both applications.
Is there a workaround that I don't see here?
Jumping over to Teams 100% is not an option as the SfB is out primary telco atm.
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Please read: https://docs.microsoft.com/en-us/microsoftteams/migration-interop-guidance-for-teams-with-skype
"Users who are homed in Skype for Business on-premises can use Teams in Islands mode but they cannot use Teams to interop or federate with other users who are using Skype for Business."
This would include Teams to Teams federation.
My guess is you have hosted SfB without Hybrid, so Office 365 is provisioning SfBO services for the Teams users vs. using the on-premises details.
To continue using Teams without interop & federation but also not break federation for your hosted SfB users you will need to sync the msRTCSIP-DeploymentLocator attribute into AAD.