The theory is that your mailboxes will be deleted after 90 days from your disconnection date. The reality is that VM often behaves in an inconsistent way in following its own processes and procedures.
VM is also currently running a programme of closing down mailboxes which are not linked to an active VM broadband subscription. This will not be helpful to anyone hoping to keep using a VM mailbox after their services have been disconnected.
You should assume some urgency in dealing with your email shutdown and set about moving your online contacts and services to another email address ASAP. Start with the most important ones first and also the ones which have the highest security (such as banking etc.) as these will likely want to sent confirmation emails to your VM mailbox when you try to switch over the email address.
If you need to back up your mailbox, Mailstore Home gets regular mentions on here and it is free
https://www.mailstore.com/en/products/mailstore-home/
I don't think there will be too much point trying to use your VM landline number as a contact if the line is going to be disconnected in a couple of days time.