"I'm sorry, the leader has left the conference."

So, you just received an incoming call, and everything seemed normal until the call connected. Then, instead of being connected with a person who was interested in talking with you, you heard a robotic voice say the following: "I'm sorry, the leader has left the conference," and the call disconnected. 

This happened because a spambot called your inbound line. While waiting to be connected, the spambot already played the recorded message that it had been designed to play to you, and disconnected the call. When you answered, the spambot was already gone, so you found yourself in a call by yourself. The robot kicked in to tell you the "leader" (the entity that initiated the call) had left, which means there's nothing more to do on the call.

The only thing left to do on your side is to select the appropriate Disposition for a bad or junk call.