To use the callback # field you first have to enter more than one number in the settings page of Google Voice. For example, it can be your Gizmo #, your home phone #, your cell phone #, work phone #, etc..

Then when you use dialcentral's callback settings it tells GV where to direct the call you start. So if you choose your home phone #, then a call you start from dialcentral will ring your home phone #.

Edit: The Google Voice webpage works the same way, when you type in a # or click a contact you want to call it will ask you which # (if you have more than one) to ring.

And see epage's post following mine.