problem after short-sale
Are you the buyer or the seller? If you paid the total due at closing I assume you're the buyer. If so why would they be calling you about the debt. If you're the seller and believe you don't owe anything more I'd verify with the mortgage company that you paid what was owed and nothing is outstanding. Was GC Services your mortage company or are they a collections agency?
GC services is a collection agency.
Why would you not send them a copy of the closing statement. I would with a cease and desist order.