Actions and anti-actions live inside conditions. Actions are executed if the condition expression matches. Anti-actions are executed if the condition don't match.
Let's look at an "hello world" example, insert the following at the beginning of default.xml dialplan:
<extension name="giovanni_05">
<condition field="destination_number" expression="^4321$">
<action application="answer"/>
<action application="log" data="WARNING YESSSSS"/>
<anti-action application="log" data="WARNING NOOOOOOOOT"/>
<action application="hangup"/>
</condition>
</extension> Then, as usual, reloadxml, and call 4321. Your terminal will show the following:
Now, call instead 5555 (or whatever):
As you can see, this time only the anti-action has been executed. There is no "answer", and no explicit hangup (the hangup there is because when there are no more (anti-)actions in the call's TODO list, then...