Oh sorry, I've made a mistake I've recently contacted Prometric for scheduling an IBM exam, the Prometric ID was enough.
For Sun it's indeed different. You have to call your local Sun representative office and ask for a voucher, this will be sent by mail, after the payment is received (snail-mail). After you have received the voucher you call Prometric or indeed book online.
You can buy the exam voucher online from the sun website - www.suned.sun.com. If you are pay by credit or debit card you can instantly get the voucher number. Using this voucher number you can book the exam online at promteric website - www.2test.com