ALT UNDERSHOT; ALT XING RESTR NOT MET BY A B737-300 FLT CREW WHEN THE PIC IS OFF ATC FREQ; ON COMPANY RADIO; WHILE XING HIGH OVER THE VOR AT CVG.

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WE WERE CRUISING AT FL270 AND WE WERE GIVEN A XING RESTR OF FL240 AT CVG. THE XING RESTR WAS ENTERED INTO THE FMC AND SET IN THE ALT OF THE AUTOPLT. THE CAPT WENT OFF FREQ TO GET OUR ARR GATE. I HAD THE PROGRESS PAGE PULLED UP ON THE FMC INSTEAD OF THE DSCNT PAGE; WHICH I NORMALLY USE. I CAUGHT THAT I WAS ABOVE DSCNT PROFILE AND NEEDED TO START MY DSCNT WHEN THE ZOB CTLR ASKED US OUR ALT. I HAD STARTED MY DSCNT IMMEDIATELY; BUT I MISSED THE XING RESTR BY 1500 FT OVER CVG AND WAS AT FL240 1 MI PAST CVG. THERE WERE NO TCASII ADVISORIES OR ALERTS. NO DEVS OFF COURSE WERE REQUIRED. THE CTLR DID NOT SAY ANYTHING ELSE ABOUT THE XING RESTR. NO OTHER SIGNIFICANT EVENTS NOTED. IN THE FUTURE; I WILL HAVE THE DSCNT PAGE PULLED UP ON THE FMC; AS WELL AS DOING THE MENTAL DSCNT CALCULATIONS. I WILL ALSO BE MORE VIGILANT WHILE THE OTHER PLT IS OFF FREQ. SUPPLEMENTAL INFO FROM ACN 638794: THERE WERE NO TA'S OR RA'S AND NOTHING WAS SAID TO ATC. I SHOULD HAVE BEEN MORE AWARE OF THE PF'S ALT AND DSCNT POINT. IN ADDITION; I WAS OFF FREQ TOO LONG AS CMH WAS NOT ANSWERING THEIR RADIO. I SHOULD HAVE COME BACK ONTO FREQ SOONER.