Good day. There are two concepts: an active call (right here and now) and an event (the history of the fact that the call was once). The call that you create is needed to show the call window in real time to the user who is being called in the box. To close the call, you can send the same request, only with a different status, for example ANSWER or CANCEL. So here you did everything correctly.
Further, all actions work with events, so in order for the action "Convert call to process" to work, you need to create an event.
https://swagger.1b.app/#/event/event