Getting bills after cancelling

I cancelled our VM services back in Feb 2024. VM sent me a letter confirming cancellation and a package to send the router back which I did and VM confirmed they received it. I then received a bill labelled final bill sometime in March which was duly taken from my account by direct debit. However the next month I get another bill for a random £21 pounds. When I rang up to understand what this was for I was told I hadn’t passed security and so they can’t talk to me about it even though I used the same answers as I’ve always done when I was a customer, and I need to try again later.

This has now happened 3 times and I’m getting increasingly concerned it’s going to affect my credit score and I can’t even find out what the bill is for! Please could someone help me.