Return of phone upon death of account holder

I tried to return the phone upon closing my sister's death Xfinity account. Xfinity took all the equipment except the phone. They said a return label would be sent to her. It has never been received and I still have the phone. In the bereavement support page it indicates "If they are returning the device, the remaining balance on the device payment plan will be forgiven." However, emails continue to be sent because payment could not be processed. Please advise on how to return this phone and finally close this account for good.