TKOF ABORTED DUE TO IMPROPERLY SET WING FLAPS. FO AND PIC MISSED ITEM ON CHKLIST.

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TAXI FOR TKOF FROM PHL FOR RWY 27L. CALLED FOR FLAPS 5 TAXI CLRNC. FO WAS RECEIVING CLRNC FOR TAXI. FREQ WAS BUSY AS WE TAXIED OUT FOR TKOF. CHKLIST WAS CALLED COMPLETE AS WE WERE CLRED FOR TKOF. AS PWR WAS ADDED; WARNING HORN SOUNDED AND CAPT TOOK ACFT AND PERFORMED LOW SPD REJECTION AT ABOUT 50 KTS. WE CLRED RWY AND PERFORMED REJECTION CHKLIST. FLAPS WERE NOT IN THE 5 DEG POS WHICH CAUSED WARNING HORN. I SUSPECT THAT FO WAS BUSY ON RADIO AND SKIPPED THE FLAPS CALL ON CHKLIST AND I; AS CAPT; DID NOT VERIFY 'FLAPS 5' AS THIS RESPONSE WAS NEVER CALLED OUT ON CHKLIST. BOTH PLTS ARE NEW TO B737-300 AND THE INTERNAL AND EXTERNAL CHKLIST AND RADIO RESPONSES AT TIMES SEEM SATURATED COMING FROM A 3 MAN CREW ON THE B727. SOMEHOW THE FLAPS WERE MISSED ON 2 CHKLISTS. I; AS CAPT; ASSUME THAT WHEN CHKLIST IS SAID TO BE COMPLETE -- IT IS COMPLETE. THIS WAS NOT TRUE. THE TKOF WARNING HORN PREVENTED ANY FURTHER PROGRESS. I WILL NOW MAKE SURE FLAPS ARE SET AS PART OF MY 'PERSONAL' CHKLIST. THE PROB HERE IS A CHKLIST WAS SAID TO BE COMPLETE BY FO AND IN FACT IT WAS NOT! THE OLD QUESTION HOW TO MAKE SURE CHKLISTS ARE IN FACT COMPLETE REMAINS A HUMAN FACTOR PROB. MY PERSONAL SOLUTION IS TO SLOW DOWN THE CHKLIST EVOLUTION IN BUSY GND OPS. THE TKOF WARNING HORN HAS BEEN INSTALLED BECAUSE OF THIS PROB AND REPEATED OCCURRENCES.