No such corporation shall terminate service to a customer because of the customer's failure to pay for service unless the corporation has first offered a monthly installment plan that is just and equitable. Each corporation must offer a just and equitable monthly plan to all customers whose service has been disconnected for nonpayment. To the extent practicable, the monthly installment plan should be based on the customer's ability to pay, past payment history, the amount of indebtedness, and the availability of other resources. The monthly installment plan should require a customer to pay current bills and a portion of past indebtedness. A new monthly installment plan need not be offered to a customer who is in default on an existing one.