A B757 FLC LEAVES THEIR PREVIOUSLY ASSIGNED ALT OF FL350 WHEN THEY POSSIBLY ANSWER TO A WRONG CALL SIGN FOR A LOWER ALT NEAR AND; SC.

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LEVEL AT FL350. ATC SAID; 'ACR X; DSND AND MAINTAIN FL310.' CAPT RESPONDED; 'ACR X; OUT OF FL350 FOR FL310.' WHILE DSNDING; ATC SAID; 'ACR X SAY YOUR ALT.' CAPT RESPONDED; 'ACR X PASSING FL325 FOR FL310.' ATC SAID WE WERE NOT GIVEN A CLRNC TO FL310. THEN ATC SAID 'ACR X DSND AND MAINTAIN FL310.' CAPT SAID 'ACR X DSNDING TO FL310.' POSSIBLY ONE OR MORE OF MANY REASONS. 1) POSSIBLY; ATC 'MEANT CLRNC' FOR ANOTHER ACFT; BUT VERBALLY SAID OUR CALL SIGN. 2) POSSIBLY; ATC ACTUALLY GAVE CLRNC TO ANOTHER ACFT; AND THE CAPT INCORRECTLY READ BACK THE CLRNC; THINKING IT WAS FOR HIS ACFT. IF THIS WAS THE CASE; THEN ATC FAILED TO NOTICE AN INCORRECT 'CALL SIGN' READING BACK HIS CLRNC; THAT WAS INTENDED FOR A DIFFERENT ACFT. IF ATC USED INCORRECT CALL SIGN; (FOR A CLRNC MEANT FOR A DIFFERENT ACFT); ATC SHOULD DOUBLE-CHK FOR CORRECT CALL SIGN USAGE. ALSO; UPON CREW MEMBER READING BACK A CLRNC TO ATC; LISTEN CLOSELY FOR CORRECT CALL SIGN READING BACK CLRNC. FOR CREW: LISTEN VERY CLOSELY FOR WHOM A CLRNC IS INTENDED. CONTRIBUTING FACTORS: THE CAPT DID NOT 'QUESTION' ATC'S CLRNC; BECAUSE THE TIME/POS WAS NORMAL FOR SUCH A DSCNT. ALSO; THE FO DID NOT QUESTION THE CLRNC; OR THE READBACK; EITHER.