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Calls succeed, but chat fails: the ACS user joins, is admitted, and appears in the roster, yet never becomes a meeting-chat member. Microsoft’s interop guidance says an admitted ACS participant should be added to that chat, so repeated 403 responses from sendMessage and typing indicate a service-side attachment or authorization failure, not normal client behavior. Confirm the organizer’s meeting-chat policy is Enabled rather than EnabledExceptAnonymous, and verify the meeting option permits chat for anonymous participants. Reproduce once with the current ACS Chat and Calling SDKs and an identity. Preserve the meeting thread ID, ACS resource and region, organizer tenant, SDK versions, UTC time, correlation vector, and policy output. Do not try to force-add the ACS identity; no supported client API performs that operation for a Teams meeting thread. Your Azure support case is the next step. Ask support to correlate the 403 with thread-membership provisioning beginning August 24.