B767-4300 FLT CREW ASSIGNED HDG; ALT AND FREQ CHANGES ON APCH TO IAD; ONLY TO BE INFORMED AS BEING AT THE WRONG ALT BY PCT CTLR.

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WE WERE ON THE JASEN ARR INTO IAD; INSIDE OF DOCCS INTXN TOLD TO DSND TO 8000 FT. I SAW TCASII TFC IN FRONT OF US AT 7000 FT. IN THE DSCNT (NEAR LDN; I THINK); TOLD TO TURN R TO A HDG (I THINK IT WAS 010 DEGS) FOR DSCNT. THEN TOLD TO DSND TO 5000 FT AND LOOK FOR VFR TFC AT 4500 FT. ON TCASII; SAW TFC AT 4500 FT. AS WE PASSED TFC; TOLD TO CONTINUE DSCNT TO 4000 FT. PASSING 5500 FT; 2 DIFFERENT ACFT TRIED TO CHK IN WITH POTOMAC. CTLR DID NOT ANSWER THEM; BUT TOLD US TO SWITCH TO A NEW FREQ; THEN LIKE HE WAS TALKING TO SOMEONE ELSE SAID; 'JOIN LOC.' WE WERE HDG ABOUT 110 DREGS OFF FROM LOC COURSE; SO THE CAPT ASKED THE CTLR IF HE WANTED US TO JOIN THE RWY 12 LOC. THE CTLR CAME BACK AND SAID; 'YES; TURN TO HDG 090 DEGS AND JOIN THE RWY 12 LOC; SWITCH TO XXXX.' BY THEN; WE WERE PASSING THROUGH THE LOC AND I WENT TO 25 DEG ANGLE OF BANK TO THE R. WE THEN GOT A TCASII 'TFC; TFC.' THERE WAS TCASII TFC N OF THE RWY 12 LOC AT ABOUT 4000 FT. I DO NOT REMEMBER THE CAPT CHKING IN WITH THE NEW CTLR BECAUSE OF EVERYTHING HAPPENING AT ONCE. BUT THE NEW CTLR SAID 'ACR X; MAINTAIN 5000 FT. YOU WERE ONLY CLRED TO 6000 FT; TURN R TO A HDG OF 150 DEGS.' WE WERE AT 4700 FT AND ALREADY IN A SHARP TURN TO THE R. THE CAPT TOLD CTLR WE WERE CLRED TO 4000 FT; BUT CTLR CAME BACK AND SAID NO; IT WAS 6000 FT. THE CAPT TOLD CTLR 2 OR 3 TIMES; 'NO; IT WAS 4000 FT;' I THINK THAT IF THE FIRST CTLR HAD GIVEN US AN EARLIER TURN TO JOIN THE LOC; WE NEVER WOULD HAVE GOTTEN CLOSE ENOUGH TO THE OTHER ACFT TO GET THE TCASII WARNING. DID THE FIRST CTLR GIVE US A NEW HDG TO FLY AND MAYBE EVEN A NEW ALT TO STOP DSCNT AT; BUT WE DID NOT HEAR IT BECAUSE OF OTHER ACFT CHKING IN?