Collide, but not react

I've got objects with collision physics and an invisible mask which needs to get a CollisionCallback to the objects ... but without setting their physics off. I tried setting the mask to material with no density or friction, but that didn't seem to do anything. Any combination of setting the collision vars seems to turn them both off.

What's the distinction between setCollisionActive and setCollisionPhysics if it isn't to control this, or am I just blindly missing something?