Citibank Debt Collection Complaint

Citibank Debt collection department,

Credit card Cont'd attempts collect debt not owed Debt is not mine Massachusetts

Citibank is calling my - yr. old grandparent 's house about 6 times a day. They are on a limited income. They have spoken about lowering the payments but, they continue to call. It 's very upsetting to them. I 've tried to speak to them myself but they are very rude and then they hang up. What can I do to resolve this issue.