calls.go
func (c *CallService) Cancel(sid string) (*Call, error) {
	data := url.Values{}
	data.Set("Status", string(StatusCanceled))
	return c.Update(context.Background(), sid, data)
}
func (c *CallService) Cancel(sid string) (*Call, error) {
	data := url.Values{}
	data.Set("Status", string(StatusCanceled))
	return c.Update(context.Background(), sid, data)
}
// Cancel an in-progress Call with the given sid. Cancel will not affect
// in-progress Calls, only those in queued or ringing.