They will not give the discount over the phone.
You cannot get the discount on bookings/purchases you make prior to boarding the ship.
If you wish to pre-book for Pinnacle, you will have to pay the full price and the discount will be applied when you actually have your dinner in Pinnacle. We have never had any problem getting our Four Star Mariner Discount but I have read of a few people who did not see it automatically applied to their on board account.
I suggest you get a print out of your bill from Front Office or Concierge in Neptune Lounge if you are in a Deluxe Veranda Suite or Penthouse and verify the discount appears on your account.
I'll anticipate the next question someone might have and add:
If you make any purchases for excursions etc or dinner reservations, anything you pay in advance of boarding will be credited as 'bonus days' added to your Mariner Account same as though you made the purchase while aboard.