Try using the AssociatedActorTag on the volume actor and the Tag event on another derived actor.
Add this class to your code:
Then add a volume to your map with the AssociatedActorTag property set to something like "VolumeHandler001"
class VolumeHandleActor extends Actor placeable;
event Touch(Actor Other)
event Untouch(Actor Other)
Finally add a VolumeHandleActor with a Tag property of "VolumeHandler001"
Note AssociatedActorTag can be found under the Volume section and Tag can be found under the Events section
When the volume's PostBeginPlay is called it will try to find an Actor in the map that has a Tag property that matches the volume's AssociatedActorTag. In this case it will find the VolumeHandleActor. Then it will call the Touch & Untouch events on that VolumeHandleActor when another actor (such as a pawn) touches the volume.
Hope that makes sense..