CLEARED 15000' CAPT THOUGHT CLEARED 11000'; SET IN ALT ALERTER.

Narrative

WE WERE ON RADAR VECTORS LEVEL AT FL310 WHEN CENTER GAVE US CLRNC TO DSND TO 15000' AT PLT'S DISCRETION. IN THE SAME XMISSION CENTER ALSO GAVE A HDG CHANGE TO INTERCEPT THE ILS AND THE ALTIMETER SETTING. WHEN I WAS COPYING THE CLRNC DOWN ON MY SCRATCH PAD I COULD HEAR THE CAPT RESETTING THE ALT ALERTER. I READ BACK THE CLRNC TO THE CTLR. THE CAPT TURNED TO THE NEWLY ASSIGNED HDG AND INITIATED THE DSCNT. I TOLD CENTER THAT WE WERE LEAVING FL310. I THEN TOLD THE CAPT THAT I WAS GOING TO BE OFF THE ATC FREQ TO SWITCH TO #2 RADIO AND CONTACT RAMP FOR WX AND GIVEN THEM AN ETA. THE CAPT ACKNOWLEDGED. I GOT THE WX AND THEN MADE A PAX ANNOUNCEMENT TO THE PAX WITH A WX UPDATE; ETA AND 'GOODBYE.' I THEN SWITCHED BACK TO #1 RADIO AND TOLD THE CAPT THAT 'I'M BACK WITH YOU.' NEXT THING I KNOW; CENTER CALLS AND ASKS OUR ALT. I TOLD THEM 'WE'RE OUT OF 11800 FOR 11000;' WHICH WAS IN THE ALT ALERT WINDOW. CENTER CAME BACK AND SAID THAT OUR CLRNC WAS 15000'. I ASKED HIM WHAT HE WANTED US TO DO AND HE SAID WE ARE NOW CLRED DOWN TO 10000'. THE CAPT HAD MISUNDERSTOOD THE CLRNC AND SET THE ALT ALERTER TO 11000'. I COULD HEAR HIM RESET IT WHEN I WAS COPYING THE CLRNC DOWN AND I NEVER CONFIRMED THE ALT SET IN THE ALT ALERTER. AFTER I HAD GOTTEN THE WX AND MADE MY PAX ANNOUNCEMENT; I JUST ASSUMED THAT WHEN I WAS OFF THE ATC FREQ WE HAD GOTTEN A CLRNC DOWN TO 11000' AND I DIDN'T QUESTION IT. THIS WOULDN'T HAVE HAPPENED IF I WOULD HAVE CONFIRMED THE ALT IN THE ALT ALERTER INITIALLY OR IF THE CAPT WOULD HAVE LET ME (PNF) DIAL IT IN WHICH IS THE STANDARD PROC. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR GAVE ADDRESS INFO AND ADVISED NO ACTION HAS BEEN TAKEN BY THE FAA. CAPT TALKED WITH CTLR AND WAS ADVISED NO CONFLICT RESULTED.