Payoff a device

Xfinity keeps attempting to collect the balance on a device I purchased. I have switched to another server. And I called several times about making payments. Since I can not afford to pay the total due all at once. No one has been able to help me in any way. I did not authorize Xfinity to attempt to collect the total amount. But I am perfectly willing to pay the balance in installments. How can I get a payment arrangement?