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Solution

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.

 

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