Unless this is for Broadband, and though you don’t say, it seems like it might not be, you don’t speak to them, not over a telephone anyway.
What you need is to get onto Live Chat and have a text conversation with them.
Details of how to access Live Chat are in line 2 of my sig, below.
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.