If I remember right, plat and cent have the same concierge. The only difference is the way the agent addresses the places they call. As an example, if you want a reservation at XYZ restaurant as a plat cardholder they say "hello, this is United747 from American Express Concierge. Can you get a reservation for one of our clients?" and for cent they say "hello, this is United747 from American Express Centurion Card Concierge and I have one of our premiere centurion card holders that would like a reservation". I remember reading about this on some website. Don't remember where though.