The complaints procedure is too long winded to be of much use. Ofcom mandate that the PAC must be provided but don't handle individual complaints. Try again on the phone with VM and be firm but polite (as I know you have been and will be).
Superuser 2016/17 Use Kudos to say thanks Mark answer as "helpful" only when the problem is solved Please don't send me private messages unless I ask you to. I do not work for VM. The advice I give is based on my best understanding of VM policy and practice. You rely on it at your own risk.
You will have to give notice to cease, then ask for the PAC, one months bill still to pay, if you speak to retentions it should go simple, choose the option on the voice response at the start of the call
Thank you for coming back and talking to the Community, it's good to see you've had a couple of interactions on this, thank you all.
J0hn has a good idea of what's most likely going on, although the manner of the call is something I am unaware of it should be to politely remind you that there may be a month outstanding as per the cease of a SIM only deal.