DC9 CREW CLRED THROUGH OCCUPIED ALT BY ATC. TCASII TA ALERTS THEM OF TFC AT SAME ALT. TCASII RA CLB STARTED BUT; UNKNOWN TO THEM; DEP CTLR ON DIFFERENT FREQ GIVES ACFT Y OPTION IN ALT FOR AVOIDANCE. BOTH ACFT ARE NOW CLBING OUT OF 6000 FT. DC9 FO GETS VISUAL ON TFC AND TURNS; DSNDS. NO PARTY LINE; SEPARATE FREQS.

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FLYING THE VANDALIA 2 ARR TO STL. AT INTXN PETIT; LEVEL 15000 FT; GIVEN CLRNC TO 5000 FT. WHILE DSNDING ON THE 078 DEG RADIAL OF STL; WE RECEIVED TA FOR KING AIR TFC AT 6000 FT. REALIZED A CONFLICT WAS DEVELOPING BY OBSERVING THE TCASII (ALT AND CLOSURE RATE AND DIRECTION OF MOVING TFC). CAPT CALLED ATC AND ATC SAID THAT WE WERE ONLY CLRED TO 7000 FT. STARTED TO CLB AND THEN RECEIVED TCASII RESOLUTION TO CLB BUT THEN TCASII SWITCHED TO A DSCNT RESOLUTION. PICKED UP TFC VISUALLY AND BY RESPONDING TO TCASII COMMANDS AND VISUAL REF MAINTAINED ADEQUATE SEPARATION. PASSED WITHIN 1 1/2 - 2 MI AND 1000 FT; ACCORDING TO ATC. ATC PULLED THE RECORDED TAPES AND FOUND THE ATC CTLR IN ERROR. WE WERE; IN FACT; CLRED TO 5000 FT. SOME CONFUSION EXISTED BECAUSE OUR INITIAL RESPONSE TO CLB WAS MADE INEFFECTIVE BECAUSE THE CTLR GAVE THE KING AIR THE OPTION AND HE ALSO CHOSE TO CLB. BECAUSE THE KING AIR WAS ON A DIFFERENT FREQ THAN US; WE COULD NOT HEAR THAT PART OF THE CONVERSATION. THE ENTIRE SEQUENCE OF EVENTS WAS SO UNUSUAL THAT WE WERE FORCED TO TOTALLY RELY ON SELF HELP BECAUSE EVEN AFTER THE DETERIORATING SIT WAS BROUGHT TO THE CTLR'S ATTN; HE WAS STILL UNABLE TO PROVIDE ADEQUATE GUIDANCE TO AVOID THE TFC. I WAS FORCED TO RELY ON MY OWN JUDGEMENT. WITH CONFLICTING TCASII COMMANDS; A CONFLICT IN THE ASSIGNED ALTS; THE CAPT TALKING ON THE RADIO TO THE CTLR AND NO RESOLUTION BY THE CTLR; I FELT LUCKY TO VISUALLY SPOT THE TFC AND IMMEDIATELY EXECUTE A L-HAND TURN AND CHANGE ALT TO INCREASE THE DISTANCE BTWN OUR 2 ACFT. IF WE HAD BEEN IN IMC IT MAY HAVE BEEN A CLOSER CALL.