Pestering calls for an unknown debt

Hi, Note this mail is on behalf of my Dad who has alerted me to the following. Creditworx has been calling to collect a "So Called" debt that he owes. This is utter nonsense as there is no debt as i have a cancellation email with the company in question which exonerates him. Please do not persist to pester the "old age". I have tried to contact Creditworx personally to no avail as you keep me on hold for 10 minutes then end the call. I know of ****a shops that operate more efficiently regards. Also adding to this we have never been sent any official request for collection from the company, i do not believe a debt collection company to be the first line of action notwithstanding that i cancelled the principals companies service about 1.5 years ago.

Regards Sanjeev