B737-700 CREW AND OTHER ACFT HAD LESS THAN LEGAL SEPARATION IN ZHU CLASS A AIRSPACE.

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POSSIBLE ALTDEV. AT FL230; 55 MI FROM AUS; PNF (CAPT) REQUESTED LOWER. RESPONSE FROM CTR (I THOUGHT) WAS 'FLT X; DSND AND MAINTAIN 17000 FT.' WE BEGAN THE DSCNT. HE GAVE ME THE ALTIMETER SETTING. I READ IT BACK. OUT OF FL210 HE SAID (CASUALLY) 'FLT X; MAINTAIN 230.' HE SAID; 'AFFIRMATIVE; MAINTAIN FL230.' WE BEGAN THE CLB BACK UP; DISCUSSED THAT WE HAD A TARGET AT 2 O'CLOCK POS; BUT NEVER GOT A TCASII WARNING. DISCUSSED THAT THIS CTLR WAS BOTH DIFFICULT TO UNDERSTAND AND ALSO SEEMED CONFUSED. MY READBACK WAS VERY CLR. IT APPEARED THERE WAS ALSO ANOTHER ACFT ON FREQ; BUT HE NEVER MADE IT CLR THERE WAS A CONFLICT; NEVER SAID I HAD TAKEN A CLRNC MEANT FOR THE OTHER ACFT; NEVER CLARIFIED MY READBACK OR TOLD US TO STOP THE DSCNT. ALSO GAVE US NO DIRECTION IN THE CLB BACK TO FL230 (WHICH MADE US EXTRAORDINARILY HIGH FOR THE DSCNT INTO AUS). ON FINAL; I WAS GIVEN A PHONE NUMBER TO CALL ABOUT 'A POSSIBLE ALTDEV.' IN AUS I CALLED ZHU. HE TOLD ME THAT ATC IS RESPONSIBLE FOR READBACKS; AND OUR TAPE WAS BEING PULLED; BUT FROM WHAT I HAD SAID IT SOUNDED LIKE A CTLR ERROR AND WE WERE NOT REMISS. SUPPLEMENTAL INFO FROM ACN 534803: WHILE DSNDING; WE WERE GIVEN FL230. IT WAS TIME TO DSND AND THE CTLR SOUNDED (HIS COM WAS POOR) LIKE HE CALLED US TO DSND TO FL190. I WAS FLYING AND THE CAPT READ BACK 'DSND TO FL190' THEN ZHU GAVE US THE CURRENT ALTIMETER SETTING AND THE CAPT READ THAT BACK ALSO. PASSING FL210; ZHU CAME ON AND TOLD US TO CLB AND MAINTAIN FL230; THAT WE WERE NOT CLRED TO DSND. WE CLBED BACK UP TO FL231; WHICH CAUSED A TA. WE HAD THE TFC IN SIGHT AND NO EVASIVE ACTION WAS NECESSARY. AFTER BEING HANDED OFF TO APCH; WE WERE ASKED TO CALL ZHU. THE CAPT CALLED AND TALKED TO CTR SUPVR WHO INDICATED THAT THE PROB WAS ON THEIR END. AT THE TIME THERE WAS AN XYZ ACFT WITH A SIMILAR CALL SIGN; HOWEVER; THE PLT TALKING ON THE RADIO FOR XYZ WAS MALE AND MY CAPT WAS FEMALE SO THERE WAS DEFINITELY SOME DISTINCTION IN SOUND FOR THE HOU CTLR; IF WE INDEED READ BACK THE WRONG CLRNC.