LTT ACFT GIVEN 4000 FT CLRNC; CTLR THEN SAYS HE THOUGHT HE GAVE THEM 5000 FT.

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WE WERE BEING VECTORED FOR AN APCH TO RWY 18R AT DFW. I WAS PNF; AND APPROX 10 MINS PRIOR TO LNDG; I TOLD THE FO I WAS LEAVING THE FREQ TO CALL 'IN-RANGE' TO OUR COMPANY. I RETURNED TO HEAR THE CTLR SAY 'DSND TO 4000 FT.' MY FO ANSWERED THE CTLR 'LEAVING 6000 FT FOR 4000 FT' AND OUR CALL SIGN. PASSING THROUGH APPROX 4500 FT; THE CTLR TOLD US TO CLB TO 5000 FT AND MAKE A 60 DEG R TURN. HE THEN QUESTIONED WHAT ALT WE WERE CLRED TO. I REPLIED 4000 FT. HE STATED HE 'THOUGHT' WE WERE CLRED TO 5000 FT AND PROCEEDED TO RE-VECTOR US FOR THE APCH. HE THEN TOLD US THAT THERE WAS NO TFC CONFLICT (WE WERE TCASII EQUIPPED AND AGREED) AND THEN HANDED US OFF TO THE FINAL CTLR. I WOULD SAY ONE WAY TO POSSIBLY AVOID THIS IN THE FUTURE WOULD BE FOR THE PNF (ME) TO TAKE CARE OF THE NON ESSENTIAL WORK (IE; CALLING 'IN-RANGE') BEFORE WE GET BUSY IN THE COCKPIT. HOWEVER; I DID RETURN TO HEAR THE CTLR ASSIGN US 4000 FT; SO BOTH THE FO AND I BELIEVE THE ERROR WAS ON THE PART OF THE CTLR WHO SEEMED UNUSUALLY BUSY AT THE TIME.