Adt Security
Contract not cancelled after request

On 2nd September my mother requested that ADT cancel their contract with her as she had engaged the services of another security company.
We kept receiving invoices which come to my email address. A few weeks ago I replied to the address stating that we had cancelled the contract and for them to please investigate why we are still receiving invoices.
I never had any response from anyone.
Of course now she is being hassled by the ADT debt collectors for an unpaid account.

Please could this matter be sorted out and closed.
thank you