Thanks for asking question! For a Communication Services user that joins a Teams meeting as a guest user can access the meeting's chat only when they have joined the Teams meeting call.
You may refer to the Teams interop documentation to see how to add a Communication Services user to a Teams meeting call.
To further elaborate on Teams interoperability this feature is currently in private preview and to enable this feature for your Communication Services resource, please email acsfeedback@microsoft.com with:
- The Subscription ID of the Azure subscription that contains your Communication Services resource.
- Your Teams tenant ID. The easiest way to obtain this is to obtain and share a link to the Team.
Please note that you must be a member of the owning organization of both entities to use this feature.
After enabling the Teams interoperability, a Communication Services user can join the Teams call as a guest user using the calling client library. Joining the call will add them as a participant to the meeting chat as well, where they can send and receive messages with other users on the call.
Please let us know if you have further query on this.