I am not sure if I had the same issue but I had a sim card free with my broadband bundle. When trying to register the sim to virgin media mobile they kept asking my mothers maiden name. Something I did not set up. Don't bother calling customer service because they just ask for the password anyways. When buying the bundle the sales team asked to create a password. If you remember what you gave use that but in all capitals.
Hope that helps, I wasted a whole day to find this out.