When you book, mark the box that states that you already have a certificate and no points will be substracted. You can make as many reservations as you want this way, overlapping or not.

I would recommend to avoid making reservations and changes on the phone when possible. The only very few times I had problems with my bookings were due to operator mistakes or missunderstandings. There is little or none they can offer you besides what you see on the screen. Go safe online.