After years of experience, you learn. I'd call again and say, "This is Ivan's Collection Agency and you've been notified a number of times. Please pick up your work by (set a date) with payment or unfortunately we'll be forced sell the piece to recoup our expenses. If for unforeseen circumstances, you are unable to pay in full, we take payments or you may send a number of new customers our way. For each new customer, we'll give (20% or whatever you decide) off their custom framing and apply it to your account until payment is made in full. Then you may take your piece with you.

Be creative and try to help the customer. Add a little humour. It works wonders.