I agree with Trip - only book with an agent. We're always here when you need us!
But as for Carnival, when you call in, 95% of the time you're talking to someone who; 1) works in a call center, 2) has worked for the company less than six months, 3) is not a certified travel agent, and 4) has never even been on a cruise or to any of the ports. Their number one job is to sell you something.
So, if you call in and ask for your PVP and they can't/won't transfer you, then either ask for a supervisor or call back and talk to someone else. Eventually, you'll get someone who will not give you the runaround because what she's saying about PVP's is not true.