Unfortunately, in times of flight or fight, we produce hormones. So yes, as much as we like to think we can control ourselves in those situations, what our body does to our mind and breathing, etc, is out of our control during those times. Now, there are those that are trained to control themselves in this situation, but the physiological activity is still occurring. This isn’t true for the general population.
That doesn’t apply to the 911 operator. We should admire the rescuers who keep their cool in order to save emotional victims’ lives.