The purpose of the "Callback Number" in DialCentral is so you can decide which phone you want to use for the call. Google Voice will call you on that phone before connecting you with the person you are calling. It took me a little while before I figured out that DialCentral doesn't place the call directly.

Cell phone vs. landline? Well, you'd be using minutes on a cell phone unless you have one of those "fave" or "family circle" plans, in which case you could set your GV number as one of your "faves." Some here have done that.

It should work with your SkypeIn number, although I haven't tried that.

Is that what you wanted to know.

You could also set someone else's phone up as a "callback number" without their knowledge, then have Google Voice connect them to you. I did that to my wife once and it confused the heck out of her.