When calling the hotline, you will be asked by an automatic system if you are waiting for your gold or platinum card, because you rented 5 resp. 20 times a year. It seems you can still earn the status by renting a car regularly.
graraps, do you have any information concerning the t&c, you told us?
They were the contents of the letter ("signed" by Mr. Sixt) and info pack sent to me with my card.