Update call-automation.md

added preview tags + answer call failed

Author: alvin-l-han

articles/communication-services/concepts/call-automation/call-automation.md

@@ -147,7 +147,8 @@ The Call Automation events are sent to the web hook callback URI specified when
 | ----------------- | ------------ |
 | CallConnected      | The call has successfully started (when using Answer or Create action) or your application has successfully connected to an ongoing call (when using Connect action)|
 | CallDisconnected       | Your application has been disconnected from the call |
-| CreateCallFailed | Your application failed to create a call |
+| CreateCallFailed (in preview)| Your application failed to create a call |
+| AnswerCallFailed (in preview) | Your application failed to answer an incoming call |
 | ConnectFailed       | Your application failed to connect to a call (for connect call action only)|
 | CallTransferAccepted         | Transfer action has successfully completed and the transferee is connected to the target participant |
 | CallTransferFailed  | The transfer action has failed  |