ACR X RESPONSE REPLY TO WRONG CALL SIGN. TCASII RA. NON ADHERENCE TO ATC CLRNC HAD LTSS FROM ACR Y. EVASIVE ACTION TAKEN.

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WHILE FLYING AS AN MD80 FO ON ACR X FROM ONT-MSP; I WAS INVOLVED WITH ZLA IN A VERY BUSY; HIGH DENSITY TFC INCIDENT. DURING THE POMONA 6 DEP; I WAS FLYING THE DAGGETT TRANSITION AND BEEN CLRED TO 17000 FT; WHEN I HEARD THE CAPT ACKNOWLEDGE OUR CLRNC TO FL230. PASSING FL18.1; ATC ASKED OUR ALT WHICH THE CAPT READ BACK 'PASSING FL18.1.' CTR THEN SAID CALMLY 'TURN L TO HDG 320 DEGS AND DSND TO 17000 FT AND THAT WE WERE ONLY CLRED TO 17000 FT. BY THE TIME I BEGAN THE L TURN AND INITIATED DSCNT; CLB MOMENTUM REACHED ABOUT FL18.6. OUR INITIAL HDG TO DAGGETT WAS 050 DEGS WHEN THE TURN WAS STARTED. CTR RADIOED WE HAD TFC AHEAD BUT DID NOT SAY THE POS. THE CAPT OBSERVED A LARGE JET AT 11 O'CLOCK HIGH AS I BEGAN THE L TURN. HE QUERIED ATC IF THIS WAS OUR TFC WHICH ATC CONFIRMED AS 'YES.' PASSING HDG 010 DEGS AND FL180; THE TCASII SOUNDED A RA; WITH A VERBAL COMMAND 'CLB; CLB NOW.' IT COMMANDED A 1500-2000 FPM CLB WITH DSCNT ENTIRELY RED ON THE TCASII SCOPE. SINCE ATC COMMANDED A L DSNDING TURN AND THE TCASII COMMANDED A CLB; THE SIT REQUIRED A QUICK ANALYSIS. AN INTRUDER HAD APPEARED AT OUR 4 O'CLOCK AND 1 MI POS AND LASTED FOR ABOUT 5-10 SECONDS. I ADVISED THE CAPT OF THE INTRUDER AND BEGAN TO COMPLY WITH THE TCASII RA. SIMULTANEOUSLY THE CAPT WAS STILL CLRING THE L DSNDING TURN AWAY FROM THE ATC PREVIOUSLY CALLED TFC AT THE NOW 12-1 O'CLOCK POS. BEFORE I COULD COMPLETELY COMPLY WITH THE RA; THE INTRUDER DISAPPEARED FROM THE TA SCOPE. I THEN COMPLETED THE L TURN AND DSNDED TO 17000 FT. SUPPLEMENTAL INFO FROM ACN 274812: I SPOKE WITH SUPVR; ZLA QUALITY CTL; TO CLARIFY QUESTIONS I HAD REGARDING ACR X ALTDEV AND TCASII RA WHILE UNDER THE CTL OF ZLA. SUPVR STATED THAT OUR READBACK OF CLRNC TO FL230 WAS COMPLETELY COVERED BY ACR Z READBACK OF CLRNC TO FL230 AND THAT CONSEQUENTLY THE CTLR DID NOT HEAR OUR XMISSION READBACK AND CORRECTLY ASSUMED THAT WE WERE STILL AT 17000 FT AS PREVIOUSLY CLRED. MY FO AND I BOTH THOUGHT WE HAD HEARD ACR X MAINTAIN FL230 AND I READ BACK THAT CLRNC. IN FACT THE CLRNC WAS FOR ACR Z. SIMILARITY OF CALL SIGNS AND SIMULTANEOUS XMISSION OF READBACKS; IN MY VIEW; CAUSED THE INCIDENT IN THAT I RESPONDED TO THE WRONG CALL SIGN AND WAS UNAWARE THAT I HAD DONE SO AS WE HEARD NO CORRECTION FROM ATC (BECAUSE ATC DID NOT HEAR US). THE SUBSEQUENT TCASII RA WAS DUE TO OUR CONFLICT WITH AN E120 'ACR Y' COMMUTER AT FL180 WHICH; ACCORDING TO SUPVR; CAME WITHIN 3 NM OF OUR ACFT DURING OUR COMPLIANCE WITH ATC'S INSTRUCTIONS TO DSND TO 17000 FT AND TURN TO HDG 320 DEGS. OUR MAX ALT PRIOR TO DSCNT WAS FL18.4 AND AT NO TIME DID WE COME IN CONFLICT WITH Z.