Cancel a queued or active call

Cancel a call that has not finished: queued, dialing or in progress. The call ends with outcome failed_cancelled. Returns {"cancelled": true} when it was cancelled, and {"cancelled": false} for a call that already finished (or reached its own outcome first), an unknown call_id or another key's call. It never returns 404, so it is safe to retry. Cancelling is free before anyone answers and within the first 30 seconds after the callee answers; cancelling 30 seconds or more after the callee answered is charged 10 credits.