Hi @testuser7
Can normal users in your organization chat with normal users in the other organization?
Can other guest users in your organization chat with normal users in the other organization?
Guest users can chat with users from other organizations if both organizations have enabled external access for Teams.
If you want to chat with external users, please make sure both organizations have external access turned on in the Teams admin center.
Secondly,guests are added to your organization's Azure Active Directory as B2B collaboration users. They must sign in to Teams using their guest account. If the normally use Teams with another Microsoft 365 organization, they need to switch organizations in Teams to interact with your organization.
If both organizations have enabled external access, make sure you have switched to a guest account and switched organizations.
Also, I recommend contacting the administrator to use diagnostic tools that they can run within the tenant to verify possible issues that affect federation. The documentation points out how to use the diagnostic tools.
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