A B757-200 FLC ATTEMPTS TO SORT OUT SOME CALL SIGN CONFUSION ON THE PART OF THE ARTCC RADAR CTLR AT ZLA WITHOUT TOO MUCH SUCCESS. FLT LEAVES ASSIGNED ALT AGAINST ZLA'S DESIRES OR KNOWLEDGE.

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THE PROB I BELIEVED OCCURRED DUE TO A MISUNDERSTANDING OF CTLR INSTRUCTIONS AND NOT LISTENING TO THE PLT READBACK. OUR ACFT WAS FOLLOWING ANOTHER TURBOJET (A320?). I AM USED TO PAYING ATTN TO ALL RADIO CALLS THAT USE OUR COMPANY CALL SIGN 'XYZ' AND THEN LISTENING FOR MY PARTICULAR NUMBER. IN THIS CASE; I THINK THE CTLR USED ANOTHER COMPANY CALL SIGN FOLLOWED BY MY FLT NUMBER. 'XYY 1234; CLB AND MAINTAIN FL290.' SINCE I DIDN'T HEAR 'XYZ' THERE WAS A LONG PAUSE AND I CALLED THE CTLR AND ASKED 'ZLA; WAS THAT FOR XYZ 1234?' HIS RESPONSE WAS 'ROGER XYY 1234; CLB AND MAINTAIN FL290' AGAIN A PAUSE (NO OTHER ACFT RESPONDING; SO I THOUGHT HE HAD CONFUSED 'XYY' WITH 'XYZ' SO I REPLIED 'XYZ 1234 CLBING FL290' PAUSE; NO RESPONSE FROM OTHER ACFT. WE THEN BEGAN A CLB TO FL290. AT AROUND FL248 THE CTLR GAVE A 'TURN HDG 100 FOR TFC' THEN A FEW SECONDS LATER SAID 'XYZ 1234 YOU WERE CLRED TO FL240; MAINTAIN FL240.' I REPLIED THAT WE HAD CHKED TWICE ABOUT THE CLB AND WE WERE CLRED TO FL290. THE CTLR GAVE US A PHONE NUMBER TO CALL LATER. 3 HRS LATER WITH ZHU (132.4) WE WERE TOLD 'SIT IS RESOLVED AT ZLA; YOU DON'T HAVE TO CALL BACK TO ZLA.' I THINK THE CTLR INTENDED TO CLB AN XYY FLT; BUT USED OUR FLT NUMBER. I HEARD THE NUMBER; BUT NOT THE FIRST PART SINCE HE DIDN'T USE 'XYZ.' WHEN I CALLED BACK TO CONFIRM; HE THOUGHT THE XYY FLT WAS TALKING TO HIM; EVEN THOUGH I WAS USING MY XYZ CALL SIGN. HE THEN CLRED WHAT HE THOUGHT WAS HIS XYY FLT; BUT AGAIN USED OUR FLT CALL SIGN. AGAIN I READ BACK XYZ CLRED TO FL290. HE DIDN'T CORRECT THE ERROR WHEN I TWICE CALLED TO INQUIRE IF THE CLB WAS FOR XYZ 1234. IN ALL CASES; THERE WAS A LONG PAUSE; SO IF THERE WAS AN XYY 1234; THEY COULD HAVE ANSWERED. THIS MUST HAVE SET OFF AN ALARM SINCE THE PLANE WE WERE FOLLOWING WAS AROUND 9 MI AND WE MATCHED ALTS FOR A SHORT TIME. SOLUTION: WE SHOULDN'T HAVE STARTED OUR CLB UNTIL WE HEARD BOTH XYZ AND THE CORRECT FLT NUMBER. THE CTLR SHOULD HAVE LISTENED TO OUR READBACKS USING THE XYZ CALL SIGN.