For Unity 3.x the behavior of the call backing out of the subscriber conversation based on inactivity is to go to the opening greeting call handler which, in turn, defaults to going to the operator call handler during the day time and the after hours interview handler at night.
In Unity 4.x you can dictate where subscribers are sent when they exit their mailbox on a per subscriber basis (BulkEdit can help you change them en mass) - including just hanging up if they back out or explicitly exit out. Probably kind of harsh but you could do that.
IN 3.x, however, you have to make some decisions about what to do with the opening greeting. One way to handle this is to add another routing rule such that all outside callers go to a different call handler than the opening greeting call handler created by setup - then you can change the opening greeting call handler to not send callers to the operator during the day as the after greeting action - you could set it to hangup after saying something like "if you want to try an extension you may do so now... goodbye". Outside callers are still treated as normal since they go through a different handler now.
It's a little annoying that the conversation is hard coded to go to the specific opening greeting call handler but unless you get to 4.0(1) or later you're stuck with this method of dealing with the problem.
