Oneplan claims outstanding payment on a closed account.

I used to have a One plan health insurance cover. The waiting period was explained to me particularly for the Dentist and Optometrist visits. I was happy to wait for these other benefits whilst I used the GP visits twice. I had to download an app where I could attach my invoice from the GP. So when the waiting period was over I wanted to claim for a dentist visit however I was not allowed. I contacted One plan regarding this and there was no clear answer as to why my request was rejected. They checked my debit order payments and found that it was up to date. I then requested to cancel this cover as it was no use to me if I could not get approved for visits. The consultant again confirmed that my account is up to date and it will be cancelled. The cancellation was done and my account was no longer debited by Oneplan. My problem is that now I am receiving calls, text messages and emails that Ii have been handed over for an outstanding amount on my Oneplan account. I have send emails and advised the people who called me to check with Oneplan and retrieve the call recordings where the agent who cancelled my account confirmed that it was up to date. This has not happened as I am still receiving the communication that I owe them. I would like to request that this be rectified as my account was up to date and has never had an unpaid debit order . I would like to request that this matter be followed and be resolved.