ACR DSNDS BEFORE REACHING PROPER INTXN ON BC APCH.

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WE WERE CLRED FOR THE LOC BACK COURSE RWY TO CLT. THE APCH SPECIFIES 4000 FT UNTIL ATELL INTXN THEN DOWN TO 2700 FT UNTIL LECAR. WE WERE USING THE LNAV PORTION OF THE FMC; I WAS FLYING WHEN BACK COURSE WAS INTERCEPTED. I WAS LOOKING AT FMC. IT SAID APPROX 6.0 DME TO CLR; (THE FMC DID NOT SHOW ATELL IN PRIOR TO LECAR) SO I STARTED MY DSCNT TO 2700 FT. THE CAPT THEN DECIDED TO PUT THE CLT VOR/DME ON HIS SIDE WHICH SHOWED WE WERE STILL APPROX 1 MI OUTSIDE OF ATELL. AT THIS POINT I WAS APPROX DSNDING THROUGH 3700 FT AND STOPPED MY DSCNT ONCE I REALIZED THAT WE HAD IN FACT NOT PASSED ATELL. UPON REACHING ATELL I THEN PROCEEDED DOWN TO 2700 FT MSL WHICH IS THE MINIMUM ALT SPECIFIED UNTIL PASSING LECAR. NO EVASIVE ACTION WAS TAKEN. WE DID NOT GET ANY GPWS; NO LOW ALT ALERT NOR WAS ANY COMMENT MADE BY CLT APCH CTL. THE ONLY OTHER PECULIARITY WAS THE CAPT HAD TRIED 2-3 TIMES TO CONTACT CLT TWR; AND IT WASN'T UNTIL ABOUT THE 3RD XMISSION THAT CLT TWR ANSWERED. CLT TWR THEN CLRED US TO LAND ON RWY 23. TO PREVENT AN OCCURRENCE OF THIS NATURE FROM HAPPENING IN THE FUTURE; WHEN UTILIZING THE LNAV PORTION OF THE FMC; 1 PLT (PNF) SHOULD BE ON 'RAW DATA;' CLT VOR AS A XCHK. THE CAPT WAS 'LATE' IN PUTTING IN THE CLT VOR/DME HAD IT BEEN PUT IN PRIOR TO EXECUTING THE APCH; WE WOULD HAVE HAD A VIABLE BACKUP DME TO XCHK WITH.