Mis-sold phone - final demand letter from ID - debt collectors now chasing an 87 stroke victim

My 87 year old mother was sent a ‘Final’ demand letter for missing a direct debit payment.  She has had no prior letters from ID regarding this.

My mother was mis-sold a phone by Carphone Warehouse in September 2019: instead of upgrading she was sold a new phone and a new contract.  Since September 2019 she has been paying for 2 contracts.

We have been in touch with Carphone Warehouse regarding this and they said it was mis-sold.  She is classed as vulnerable.  In September she would have been suffering from a slow bleed on the brain.  She had a stroke in late January 2020 and is still recovering.  She is now being chased by debt collectors for an outstanding sum of 2 months.  Fact: she has been paying a direct debit for a year for a phone she didn’t realise she had.  To make matters worse she will also have to pay a further 1 month so that she can take out the 30 day cancellation.  

I have used ‘chat’ but didn’t get a resolution.  Carphone Warehouse have used ID chat - no resolution. Can somebody look into this please as it is causing my mother undue stress and hindering her stroke recovery.  Thank you