B727. ARTCC RADAR CTLR MISSPOKE THE ALT CLRNC.

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CLBING ON THE LAST CLRNC TO CLB TO FL290; I TOLD THE FO TO REQUEST FL330 AS A FINAL CRUISING ALT THE NEXT TIME HE TALKED WITH ATC. AT THE NEXT FREQ CHANGE; HE REQUESTED FL330 AS A FINAL. I BELIEVE A FEW MOMENTS PASSED. THEN ATC SAID '(OUR CALL SIGN) CLRED TO FL330 I HAVE YOUR REQUEST.' THE FO READ BACK '(CALL SIGN) CLRED TO FL330' WITH NO OBJECTION FROM ATC. JUST BEFORE REACHING FL300; ATC SAID '(CALL SIGN) YOU WERE CLRED TO FL290; SAY ALT.' THE FO REPLIED 'WE WERE CLRED TO FL300.' (I BELIEVE HE MEANT TO SAY 330.) I PICKED UP THE MIKE AND SAID 'WE ARE DSNDING FROM FL300 TO FL290; WE UNDERSTOOD WE WERE CLRED TO FL330 (CALL SIGN).' WE LEVELED AT FL290 SHORTLY AFTER THAT. IN RETROSPECT; IT WOULD HAVE BEEN BEST IF I WOULD HAVE QUESTIONED A CLRNC WHERE ATC SAID 'I HAVE YOUR REQUEST' -- EVEN WHEN THE WORDS 'CLRED TO FL330' WERE SAID BY ATC. IT ALSO WOULD HAVE BEEN BETTER IF ATC MEANT FOR US TO LEVEL AT FL290 TO SAY 'CLRED TO FL290; I HAVE YOUR REQUEST FOR HIGHER.' REGARDLESS OF THE PHRASEOLOGY THAT IS CHOSEN BY ATC; IF A FLC READS BACK A CLRNC NOT CONSISTENT WITH THE INTENT OF THE CTLR; I HOPE IT WOULD BE CAUGHT AND THE CLRNC CLARIFIED.