The user in second domain could be in Islands mode, SfBOnly mode, SfBWithTeamsCollab mode or SfBWithTeamsCollabAndMeetings mode.
The following image shows the federated routing for new chats or calls. The red box highlights your scenario. SfB* represents any of the following modes: SfBOnly, SfBWithTeamsCollab, SfBWithTeamsCollabAndMeetings.
For more information, please refer to the ** Federated routing for new chats or calls** section in this link.
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