82 year old mother charged for modem after turning it in a year ago

My 82-year-old mother had a "Mystery Charge" on her bill a month ago.  She called Cox Billing Support, which is quite a time commitment, and was told by Kyle he was removing the charge from her account, gave her a confirmation number, and told her the next bill would show that all she owed Cox was her monthly charge.  Next bill comes, and it says she has a past due on the "Mystery Charge".  So today, we call Cox together.  Now the charge is for a modem we turned in a year ago when we switched her to Panoramic Wi-Fi.  We explained that we did not have the Cisco telephone modem as we turned it in when she began renting the Panaromic Wi-fi.  After trying to get the gal to understand that the Panaramic Wi-Fi served as a telephone modem, she told me "you have the Cisco modem, I can see that it's on now."  It wasn't , it isn't.  Checked every cox plug in in the house.  Not suprisingly, we did not have two modems connect to the cox service.  I was explaining this and was disconnected.  Called back.  I asked the new Customer Service rep if she could see a Cisco modem live on the system from my Mom's residence.  No.  So I explained that we had a confirmation number saying this charge was being removed from the system.  Suggested they pull up the recording of the conversation because they had "no record" of the credit.  No can do.  So now she said we needed to return the modem.  I again stated that the modem was returned to the local store when we picked-up the Panoramic modem.  Do you have the 1 inch x 1 inch receipt a year later?  No.  Who keeps these tiny scraps of paper.  I asked if Cox showed my mom still having the modem, why they didn't contact us before now.  No answer.  I explained there was no reason for us to keep the modem.  They have no value.  But, I was told that without the 1 inch x 1 inch receipt there was nothing they could do for a 15-year-plus customer.  So I asked when my Mom asked for her phone service to be switched from a land line to digital service, because she didn't.  Cox forced her to switch to digital.  Knowing she hadn't request the switch, I asked if I could get a copy of my mom's request that they install a modem.  Said they couldn't.  So I asked for a copy of what she signed saying she had received the modem.  Said they couldn't.  So I said then what proof do you have that she ever had the modem.  And then I said, you know what, I'm going to trust you and believe that at one point this modem was in the house.  Why can't you believe me when I assure you that the modem was returned?  The modem has no value to my mother.  I don't even think Cox still uses this modem.  Neverthless, my mother has spent days looking for it despite the fact that I confirmed with her that she did indeed return it - I was with her.   Thank you Cox for hassaling my elderly mother and sending her to bed with tears on multiple occasions.  Thank you for charging my mother $122 for a modem that sells for $20.  Thank you for charging a customer for outdated hardware that you will not use.  Thank you for holding your customer responsible for bad record keeping.  Thank you for neglecting to add equipment check-out and returns on your monthly bills so that customers have some way of knowing your service team neglected to enter the return or that record did not transfer to your master data system.  Thank you for expecting your customers to hold on to 1" x 1" receipts to the end of time.  Thank you.