Why did Verizon ruin our credit score?

For five years my wireless internet provider was Verizon and we switched to Optimum online as a cable, telephone and internet provider. Optimum notified Verizon of the change and Verizon continued billing us. My husband unsuccessfully attempted to speak to a Verizon representative to correct the bill and in the meanwhile Verizon placed a 'charged off' notice to our credit report. This resulted in a fifty to sixty point reduction in our FICO score. We have repeatedly attempted to correct the billing error on Verizon's part and now we are penalized by it with a lower FICO score. Our credit report clearlys shows that we paid our account on time for five years. What do we do?