Shocking Customer Service after Cancellation of a Contract

Today I was called by a company Norman Bissett Attorneys for a "supposed" arrears amount on my ADT account. This comes as a complete surprise as it was in October 2020 that I discovered that ADT had not been providing a service to us and had been sending monthly invoices to a closed mailbox for years.

On discovery of the debiting, ADT agreed to close the account and even investigate the possibility of a refund which I thought would never happen but was happy for them to do.

Fast forward 10 months and this clearly was not done and horrified to be called by a legal company. They advised I contact ADT directly for this to be dropped. Please can you assist as a matter of urgency.