Best answer by Mohammed
Welcome to our community!
You can quite easily do this from within the iD App or account as it's been pointed out above.
You'll need to first register for an iD account by downloading the app or going to https://my.idmobile.co.uk/
Once registered and signed in, just head to My Services on the side menu and flick the switch for Voicemail off/on.
I've attached some pictures below, hope this helps!