Tormented by MBD for a non existent Mixtelematics debt

I had Mixtelematics Beamie car track contract which I cancelled after 6 years of their service. I did not like the fact that they were randomly debiting my account for an agreed upon amounts more than once per month. So I gave a 30 day notice on the 1st of December 2021, so the 31st of December 2021 the account should have been closed, but it wasn't. Mixtelematics only responded mid January 2022 confirming the account cancellation.

Big problem now is that Mixtelematics decided to hand my account to MBD debt collectors in February 2022. I have explained to their numerous consultants that I do not owe Mixtelematics anything, have provided proof that Mixtelematics confirmed the account has been closed, however on daily basis I am tormented by MBD call center agents whom take pleasure in asking me to pay for a non existent contract with Mixtelematics.

I have on numerous occasions requested to speak to the MBD Management instead of escalating the call, they hang up and repeatedly call me using different numbers.