FLC OF A LEAR JET; LR25; DSNDED BELOW ASSIGNED ALT ACCORDING TO ATC CTLR WHO INTERVENED AND THEN REVISED THEIR CLRNC TO THE LOWER ALT TO WHICH THEY WERE DSNDING. BOTH CREW MEMBERS UNDERSTOOD THE ALT AS TO THEIR ACKNOWLEDGEMENT TO ATC. HOWEVER; IF IT WAS WRONG; ATC DID NOT ADVISE OTHERWISE.

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WE WERE DSNDING TO FL290 WHEN 1 CTLR HANDED US OFF TO THE ZTL CTLR WHERE THE OCCURRENCE HAPPENED. WE CHKED ON (I; THE PIC WAS FLYING) AND WE HEARD HIM ACKNOWLEDGE US AND GIVE US A CLRNC TO FL260. THE FO READ BACK THE CLRNC TO FL260. AT FL275 THE CTLR ISSUED A CLRNC OF DIRECT TO DEST ONCE AT FL270. THE FO READ BACK DIRECT TO DEST 'OUT OF' FL270 (AS WE HEARD THE ORIGINAL CLRNC WAS TO FL260). WE WERE JUST PASSING FL263 FOR FL260 WHEN THE CTLR SAID WE WERE SUPPOSED TO HAVE MAINTAINED FL270; BUT TO NOW MAINTAIN FL260. HE THEN REITERATED MAINTAIN FL260 AND ISSUED AN ACR FLT A CLRNC TO MAINTAIN FL250. I AM NOT SURE IF THE CLRNCS WERE LIMITED BUT AM JUST ASSUMING SO. WHILE PASSING FL270 WE HAD NOTICED A B727 PASS OFF OUR NOSE FROM L TO R AT ABOUT A PERPENDICULAR FLT PATH. I COULD NOT TELL WHAT AIRLINE THE B727 WAS AS IT WAS WELL BELOW US. FROM WHAT I COULD SEE; NO EVASIVE ACTION WAS TAKEN BY ANY ACFT DUE TO THIS OCCURRENCE. THE FREQ WAS FAIRLY BUSY AND JUDGING BY THE CTLR'S VOICE HE HAD A LOT TO HANDLE. BECAUSE OF THIS HE MAY HAVE ISSUED US AN ALT OTHER THAN WHAT HE INTENDED. OR HE MAY NOT HAVE HEARD A POSSIBLY INCORRECT READBACK ON OUR PART. BOTH MY FO AND I HEARD FL260 AS A CLRNC AND SO NEITHER OF US QUESTIONED THE CLRNC.