Customer service doesn't understand that an executor is an authorized user

My sister passed away several months ago and I have been appointed the executor of her estate.  I dropped off all of her rented equipment to Cox in June.   Her account was supposed to be closed.

Since then Cox has been sending texts demanding payment of several hundred dollars.  Apparently they have been billing her for the months since her death.

I have called customer service several times to see her bill but they say I am not an authorized user.  One of the executor's job is to act as the deceased person's representative.  So what they are telling me is not true and they would not let me speak to their superior.

They promised twice to send a form to me so I can send them a death certificate.  Unfortunately, this was a ruse to get me off the phone because they did not do it either time.

They suggest I go to a Cox store but I live in another state so this is not practical.

I am at a loss of what to do at this point.  Any suggestions would be appreciated.   I was able to close all of her accounts with no problem and the employees always expressed their sympathy.  The glaring exception is Cox which has done neither of these things.