Contact your voicemail provider and request they provide the service you are paying for.
You can call NOW Broadband customer services on 03303 323 050, if they are the provider here.
Set a Payment PIN on your account so that no-one but you can buy memberships on it. Check your bank accounts monthly for any other unexpected payments to Now. That way you can at least nip them in the bud, while you and Now figure out whose fault they are.