POTENTIAL CONFLICT DURING AN ALTDEV BY A DSNDING CARJ FLT CREW WHEN FAILING TO RECONFIRM ACTUAL ALT ASSIGNMENT WITH APCH CTLR AT DEN.

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WHILE ON APCH FOR DEN WE WERE ISSUED A HDG OF 350 DEGS; SPD OF 210 KTS AND AN ALT; WHICH I DID NOT HEAR. THE CAPT (WORKING THE RADIOS) READ BACK THE HDG; AIRSPD; AND AN ALT OF 9000 FT. THERE WAS NO RESPONSE FROM THE CTLR. AT THIS POINT THE CAPT TURNED TO ME AND ASKED 'HE DID SAY 9000 FT; RIGHT?' I TOLD HIM THAT I DID NOT HEAR THE ALT AND THAT I HAD ONLY HEARD THE HDG AND THE SPD. THE CAPT THEN KEYED THE MIKE AND ASKED THE CTLR TO VERIFY OUR ALT OF 9000 FT. THE CTLR RESPONDED WITH A REMARK WHICH WAS PARTIALLY BLOCKED BY ANOTHER ACFT; BUT IT ENDED IN 'FIVE ZERO.' THE CAPT RESPONDED BY SAYING 'ROGER.' I WAS CONFUSED AS TO WHAT WAS SAID SO I ASKED THE CAPT WHAT HE HAD HEARD. HE RESPONDED BY SAYING 'I THINK HE SAID ROGER OR AFFIRMATIVE.' I TOLD HIM THAT THIS WAS NOT WHAT I HAD HEARD AND THAT I THOUGHT THE CTLR WAS ISSUING INSTRUCTIONS TO ANOTHER ACFT. I ASKED THE CAPT TO VERIFY THE ALT AGAIN. HE RESPONDED BY SAYING 'HE IS GOING TO GET MAD AT ME.' THE CAPT THEN PUT HIS HAND TO THE MIKE; MAKING IT SEEM AS IF HE WERE GOING TO XMIT. DUE TO FREQ CONGESTION HE WAS NOT ABLE TO XMIT. AT ONE POINT THE CTLR ISSUED INSTRUCTION TO ANOTHER ACFT ON THE ARR TO DSND TO 9000 FT. AT THIS TIME THE CAPT TURNED TO ME AND SAID 'YEH; IT WAS 9000 FT.' CONFIDENT THAT THIS WAS OUR ASSIGNED ALT I CONTINUED THE DSCNT. AT 12500 FT THE CTLR CONTACTED US AND TOLD US THAT WE WERE 500 FT LOW. WE IMMEDIATELY CLBED BACK TO 13000 FT AND THE CTLR ISSUED US A TFC ALERT FOR TFC PASSING BELOW US. I THINK THE FACTORS CONTRIBUTING TO THIS DEV WERE POOR COCKPIT COM AND FREQ CONGESTION ON DEN APCH CTL. I ALSO SHOULD NOT HAVE DSNDED UNTIL I WAS 100% SURE; IN MY MIND; WHAT THE CORRECT ALT WAS. I ALSO SHOULD HAVE TAKEN THE INITIATIVE TO QUERY APCH CTL MYSELF; WHEN THE CAPT WAS HESITANT BECAUSE HE WAS AFRAID OF GETTING YELLED AT.