[Promoting from a comment to an answer for posterity. This ended up resolving the OP's problem]
I'll note that trying to send a message "too soon" in a teams interop call can fail with 403. The reason is that the Azure Communication Services user is added to the chat thread after they join the Teams call. So, trying to send the message before joining the call / immediately after can fail. This problem should resolve within a few seconds.
This is not usually a problem, because the application only sends a message after the end-user types something.
Looking at the code snippet you posted, it looked as if you were trying to test things out by sending the message programmatically. In that case, it is possible to send the message to soon. So just double checking.