Hello Sami
If you have been handed the same type of letter as I have, as long as you have a cruise already booked that sails before the stated date in the letter, you don't have to do anything other than present the letter at the casino cashier's desk when on board.
If you haven't, but are trying to make a booking, again, you don't have to do anything extra until you are on board.
The 0345 number on the letter merely puts you through to P & O customer services, so you must have been unlucky with the rep you connected with who should have explained this.
If your circumstances are different from my suppositions, let me know and I'll try to offer further assistance
John