MD80 CREW HAD ALTDEV.

Narrative

WE WERE INBOUND ON THE QBALL 4 ARR INTO STL AT FL280. ZKC GAVE US A DSCNT TO FL240 WITH DISCRETION TO FL180 AND TO EXPECT QBALL INTXN AT 15000 FT. DSNDING THROUGH FL260; THE CAPT ASKED IF WE COULD LEVEL OFF SINCE HE DID NOT WANT TO DSND FURTHER JUST YET. THE CTLR OK'ED FL260. I ADVISED THE CAPT I WAS OFF VHF #1; SO I COULD MAKE OUR COMPANY IN-RANGE CALL. AFTER MAKING THE CALL ON VHF #2; I RETURNED TO VHF #1 AND NOTICED THE CAPT HAD STARTED HIS DSCNT OUT OF FL260. HE HAD SET THE ALT ALERTER TO 15000 FT AND BASED ON OUR DISTANCE FROM QBALL INTXN; WE WERE JUST ABOUT RIGHT FOR MAKING THE NORMAL 15000 FT XING RESTR. I ASKED THE CAPT IF HE HAD ADVISED ATC THAT WE WERE STARTING OUR DSCNT. HE SAID HE HAD NOT AND HE THUS MADE THE CALL; LEAVING FL260 FOR 15000 FT. THE CTLR ACKNOWLEDGED. SINCE THE CAPT HAD SET THE ALERTER TO 15000 FT; MADE THE CALL DSNDING TO 15000 FT; THE CTLR ACKNOWLEDGED THE CALL; AND WE WERE AT THE NORMAL DISTANCE TO MAKE THE 15000 FT XING RESTR; I ASSUMED 15000 FT WAS OUR CLRED ALT. DSNDING THROUGH ABOUT 17000 FT; THE CTLR TOLD US TO DSND TO 15000 FT. THE CAPT RESPONDED THAT HE THOUGHT THAT WAS WHAT WE WERE CLRED TO ANYWAY; BUT I WASN'T POSITIVE OF THAT SINCE I HAD BEEN OFF VHF #1 TO MAKE THE IN-RANGE CALL. WHEN HE HAD SET 15000 FT IN THE ALT ALERTER AND HAD STARTED TO DSND. IT WASN'T UNTIL THE CTLR MADE THE DSND TO 15000 FT CALL AS WE WERE GOING THROUGH APPROX 17000 FT DID I HAVE SOME CONFUSION AS TO WHETHER WE HAD PREVIOUSLY BEEN GIVEN A DSCNT TO 15000 FT WHILE I WAS OFF VHF #1. I BELIEVE A WAY TO PREVENT SUCH CONFUSION IN THE FUTURE WOULD BE TO MONITOR VHF #1 WHEN MAKING THE IN-RANGE CALL OR ANY OTHER CALL ON VHF #2 -- ESPECIALLY ON AN ARR AND GETTING NEAR WHERE WE WOULD BE STARTING A DSCNT; EVEN THOUGH WE ARE WELL ABOVE THE 10000 FT STERILE COCKPIT ALT.