AN ACR MLG RESPONDED TO A TCASII ALERT BY LEVELING IN DSCNT.

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DSNDING INTO RIC FOR APCH; WE WERE CLRED TO 9000 FT MSL BY APCH. PRIOR TO 9000; WE RECEIVED FURTHER CLRNC TO 5000 FT MSL. THE FO ACKNOWLEDGED WITH CALL SIGN 'CLRED TO 5000 FT.' I ALSO HEARD THE CLRNC AS 5000 FT. AS WE DSNDED TO 8700 FT MSL; WE WERE ALERTED BY TCASII OF A PREVIOUSLY UNSEEN CONFLICT DIRECTLY IN FRONT OF US (LESS THAN A MI) AND 800 FT BELOW US. WE IMMEDIATELY BEGAN TO SLOW THE DSCNT AND ASKED APCH ABOUT THE TFC. APCH RESPONDED BY TELLING US THAT THE TFC WAS AT 8100 FT AND THAT WE WERE AT 8700 FT (NO CONFLICT). HIS ATTITUDE ON THE RADIO INDICATED A PROBLEM IN COM (HIS LACK OF CONCERN). WE IMMEDIATELY ASKED HIM TO CONFIRM OUR CLRNC TO 5000 FT AND WERE CUT OUT. I STOPPED THE DSCNT AT BTWN 8600 FT AND 8500 FT AS TCASII SHOWED THE UNSEEN TFC PASSING UNDER AND BEHIND US. I BEGAN A SLIGHT CLB AS WE CONTINUED TO TRY TO CONFIRM OUR CLRNC ALT. FINALLY APCH ANSWERED OUR INQUIRIES BY TELLING US OUR CLRNC WAS 9000 FT. A SHORT TIME LATER WE WERE AGAIN CLRED TO 5000 FT. DURING THIS PERIOD; AFTER CLR OF THE TFC; APCH MISTAKENLY CLRED US TO AN IMPROPER FREQ. WHEN I ASKED HIM ABOUT THIS; HE SAID THE FREQ CHANGE WAS FOR ANOTHER ACFT. THEN HE SENT THAT ACFT TO THE NEW FREQ. DURING THE TIME BEFORE THE POTENTIAL CONFLICT; WE WERE NEVER ADVISED OF ITS EXISTENCE. AFTERWARD; NOTHING WAS AGAIN SAID OF IT BY THE CTLR.