The VM forum team staff on here will refer you back to the other comm's routes for cancellations or renewals.
Usual advice on here for phoning retentions is to call first thing in the morning when the lines first open for the best chance of getting through with least delay.
Before doing that though, arm yourself with some information about other suppliers and prices so you know if anything VM offers is a good deal or not. People posting on here have found the advice from Andrew-G at the bottom of the page below to be helpful when thinking about renewing
If you just wish to leave with no further discussion or negotiation with VM then you can write in to VM as per info below
use a first class, signed-for delivery service for proof of delivery to VM to give your 30 days notice to cancel. That way you get a date/time receipt to start the 30 day count from.