Test another Telephone cable between the Modem and the Phone
1. Check the telephone cable between the Modem and handset has no kinks and is not damaged.
2. Ask the customer to test a different telephone cable between the Modem and the Telephone handset.
3. If the customer doesn't have a spare telephone cable or the existing cable is damaged, advise them to purchase a replacement RJ11 telephone cable from any electrical or hardware store.