CAPT OF AN MDT MISINTERPRETED THE DSCNT XING ALT AND CONSEQUENTLY OVERSHOT IT CAUSING ATC TO INTERVENE AND AMEND THE CLRNC.

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ON A REVENUE FLT FROM NASHVILLE; TN; TO ORLANDO; FL; I WAS THE PF; CAPT WAS HANDLING RADIOS. WE WERE CRUISING AT FL310. ZJX GAVE US CLRNC TO VALDOSTA VORTAC; THEN THE LESSEE ARR TO ORLANDO INTL. OUTSIDE OF VALDOSTA; CTLR GAVE US A DSCNT CLRNC TO CROSS VALDOSTA AT FL240. CAPT READ BACK CLRNC AND PUT FL240 INTO ALT BOX. I DIDN'T HEAR THE CTLR CLRLY ENOUGH; BUT THE CAPT CONFIRMED FL240 BY POINTING TO ALT PRESELECTOR; AND I POINTED TO IT (COMPANY PROC). I INITIATED THE DSCNT. PASSING FL260 THE CTLR SAID 'YOUR CLRNC WAS TO FL270.' I STARTED TO LEVEL OFF AT APPROX FL260 AND CAPT TOLD ZJX THAT HE READ BACK FL240. THE CTLR SAID; 'NO; YOUR CLRNC WAS TO FL270; IF YOU WANT TO DSND TO FL240 YOU CAN; BUT I GAVE YOU CLRNC TO FL270.' I CONTINUED DSCNT TO FL240. BECAUSE I DIDN'T HEAR THE CLRNC CLRLY WHEN INITIALLY GIVEN; I'M 90 PERCENT SURE THE CAPT READ BACK FL240 AND THEREFORE ALMOST CERTAIN THE CTLR GAVE US FL240. NO EVASIVE ACTION WAS TAKEN; AND WE CONTINUED THE ARR; BUT THE CTLR SEEMED ADAMANT THAT HE GAVE US FL270. HAD I QUESTIONED THE INITIAL CLRNC TO THE CAPT OR ZJX (BECAUSE I DIDN'T HEAR IT CLRLY TO BEGIN WITH); MAYBE WE COULD HAVE CLRED UP THE CONFUSION.