Let's say I am out with a partner. At the base of a climb I don't want to climb it. My partner decides to climb it, leaves his pack and takes off. At that point we are split up. He made his choice. I become responsible first for getting MYSELF out to safety. So I would start descending immediately.
Without hindsight there is no reason yet why I should be making a phone call. I don't know anything has gone wrong. All I know is that my partner has taken off on me on something I did not want to do, and I have retreated.