I would answer, and tell them I would be happy to come out and repair the freezer, BUT I cannot because they never paid the bill for the dishwasher. If they act like they did, or say they didn't realize it, tell them you need the payment for the previous bill, and UPFRONT payment for the new repair. Once the diagnosis is made, you need the money upfront before fixing it. I would also tell them that if they don't want to pay, you will be informing the other repair services in the area about them so they don't get stiffed. Worth a shot, and you might get your money for the work you did on dishwasher.