Hi there @Cw27
Welcome to our Community and thanks so much for your first post - sorry to understand that you're having some issues with your Voicemail at the moment. The PIN feature is only available for those customers who have our Voicemail Plus service and the PIN is set by the customer by calling 1571 and following the steps advised.
The PIN allows you to access your voicemail when away from home and also keeps them safe from anyone else listening to them.
I have located your account from here for you and have been able to reset the PIN for you - I'll pop you a quick Private Message so we can go through security and give that to you. Please look for the purple envelope in the top right of the page and pop me a reply when you can.
Cheers