LGT DSNDING MISSED AMENDED ALT ASSIGNMENT AND HAD LTSS WITH AN ACR MLG.

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LGT X CALLED ON FREQ DSNDING TO FL240. DSNDED LGT X TO FL220. LGT X REQUESTED LOWER. DSNDED LGT X TO 11000. DURING PLT READBACK ALT I OBSERVED DATA BLOCK OF MLG B CLBING TO FL230; PUTTING HIM IN CONFLICT WITH LGT X. INSTRUCTED LGT X TO AMEND ALT; MAINTAIN 16000. HAD CTLR WORKING MLG Y AMEND HIS ALT TO 15000. PLACED DODECAGON ON LGT X. MLG Y CALLED ON FREQ CLBING TO 15000. ISSUED TFC TO MLG Y ON LGT X. ISSUED TFC TO LGT X ON MLG Y. AT 5 NM I OBSERVED DATA BLOCKS OF LGT X AND MLG Y AT 16000 AND 15000 RESPECTIVELY. ON THE NEXT 'HIT' (UPDATE) OBSERVED ALT OF LGT X AT 15600. ASKED PLT OF LGT X TO VERIFY ASSIGNED ALT OF 16000. PLT REPLIED IN THE NEGATIVE STATING HIS LAST CLRNC WAS DOWN TO 11000. ISSUED AN IMMEDIATE L TURN TO LGT X. ISSUED AN IMMEDIATE L TURN TO MLG Y AND CALLED THE TFC ON LGT X. NO REPLY RECEIVED. REISSUED IMMEDIATE L TURN TO MLG Y. PLT REPLIED IN A L TURN THEN STATED 'TFC IN SIGHT.' LGT X RETURNED TO 16000 FT AT WHICH TIME BOTH ACFT WERE CLRED ON COURSE. I CONTINUED TO WORK THE POS UNTIL BEING RELIEVED. WE FOUND OUT LATER THAT THE AMENDED ALT CLRNC TO LGT X WAS ACKNOWLEDGED BY THE WORD 'BLOCKED' AND NOT THE WORD 'ROGER' AS I HAD THOUGHT. SUPPLEMENTAL INFO FROM ACN 254377: WHILE DSNDING; CTR CALLED OUT TFC AHEAD OF US AT 15000 FT; 1 O'CLOCK POS. THE COPLT ACKNOWLEDGED SAYING 'LOOKING FOR TFC.' AT THE TIME; WE WERE MOSTLY IMC; FLYING IN AND OUT OF SMALL BUILDUPS. AT APPROX 15300 FT CTR CALLED AND ASKED 'WHAT IS YOUR ALT.' THE COPLT RESPONDED 'DSNDING TO 11000 FT AS ASSIGNED.' THE CTLR GAVE US AN IMMEDIATE TURN TO THE L; ALSO GAVE OPPOSITE TFC AN IMMEDIATE TURN. I DISCONNECTED THE AUTOPLT; MADE AN EVASIVE L TURN AND BEGAN CLBING. WE LEVELED AT 16000 FT. WE HAD COME WITHIN 1 MI OF AN ACR MLG. I CALLED ZID AFTER LNDG. THEY REVIEWED THE TAPES. THE SUPVR TOLD ME THAT THE CTLR HAD ISSUED A DSCNT TO 11000 FT. HE ALSO SAID THE CTLR REISSUED 16000 FT; BUT THE TAPES REVEALED THE CLRNC WAS NEVER RECEIVED. INSTEAD THE PHRASE 'BLOCKED' WAS HEARD ON THE TAPE; WHICH HE CLAIMED SOUNDED LIKE 'ROGER' TO HIM. AFTER OUR FIFTH MORNING IN A ROW OF FLYING ALL NIGHT; FATIGUE MAY HAVE SET IN. I GUESS MY POINT HERE IS; WE SHOULD HAVE CAUGHT THE CTLR'S MISTAKE AND QUERIED HIM ABOUT THE ALT. IT ALSO BRINGS OUT THE ISSUE; WHY ARE CARGO PLTS UNIMPORTANT WHEN IT COMES TO TCASII ISSUES. SUPPLEMENTAL INFO FROM ACN 254694: BY THE FIFTH DAY OF FLYING; FATIGUE IS A REAL CONCERN AND PROB. THE POINT HERE IS THAT WE SHOULD HAVE CAUGHT THE CTLR'S MISTAKE AND QUESTIONED HIM ABOUT THE ASSIGNED ALT.