AN A320 CREW RPTED THAT THEY ACCEPTED THE CLRNC OF A COMPANY ACFT; WHICH WAS NOT YET ON FREQ; AND THAT HAD A SIMILAR SOUNDING CALL SIGN. THE ZLA ARTCC CTLR DID NOT CORRECT THEM UNTIL THEY GOT CLOSE TO ANOTHER ACR'S ACFT.

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ON NOV/XA/99; FLYING ACR FLT XX; SCHEDULED FLT FROM SAN TO LAS; ATC CALL SIGN 'ACR XX.' WE HAD BEEN CLRED DIRECT POM; DIRECT DAG; FLT PLAN (THE CRESO 3 ARR). CRUISING AT FL290 WE SAW A LARGE CLOUD BUILDUP AT 12 O'CLOCK POS; JUST N OF ONT ARPT. WE REQUESTED HIGHER TO CLR THE CLOUDS AND AVOID TURB. ATC WAS UNABLE. WE MADE AN ANNOUNCEMENT FOR THE PAX TO TAKE THEIR SEATS; AND TURNED ON THE SEATBELT SIGN. WE TURNED ON ENG ANTI-ICE. WE THEN REQUESTED LOWER TO TRY TO GO BELOW THE CLOUDS. ATC CLRED US TO FL270. WE READ IT BACK AND BEGAN OUR DSCNT. PRIOR TO REACHING FL270 WE RECEIVED A CLRNC TO FL240. WE READ IT BACK; 'ACR XX CLRED TO 240.' NO XMISSIONS WERE RECEIVED UNTIL ATC CALLED 'ACR XX SAY ALT;' AND WE RESPONDED THAT WE WERE DSNDING THROUGH FL245 FOR FL240. ATC RESPONDED 'ACR XX; CLB IMMEDIATELY TO FL250.' WE ANSWERED AND CLBED BACK UP TO FL250. AT THAT TIME ATC GAVE INSTRUCTIONS TO AN ACR Z TO LEVEL OFF AT FL240; OR TO STOP CLBING; OR SOMETHING TO THAT EFFECT. WE HEARD ACR Z RPT THAT HE WAS LEVELING OFF AT FL236; HE HAD US IN SIGHT; AND THAT WE WERE NOT A FACTOR. AT THIS TIME IT BECAME APPARENT TO US THAT THERE HAD BEEN A CONFLICT BTWN US AND ACR Z. THE CTLR WAS OBVIOUSLY FLUSTERED AND WAS MIXING UP SOME CALL SIGNS. WE HEARD HIM ISSUE DSCNT CLRNCS AT LEAST 3 OR 4 TIMES TO AN ACR XXY; WHO APPARENTLY HAD NOT EVEN CHKED IN ON THE FREQ. WE LATER HEARD ACR XXY CHK IN AT FL280. THE RADIO FREQ WE WERE ON WAS 125.72. WHEN WE HAD BEEN HANDED OFF TO THE NEXT SECTOR SHE ADVISED US THAT WE WERE INVOLVED IN A 'POSSIBLE PLTDEV' AND TO CALL A PHONE NUMBER. WE CALLED ON THE GND AND WERE INFORMED THAT WE HAD GOTTEN TOO CLOSE TO ACR Z; WE HAD BEEN ASSIGNED FL270; AND WE HAD APPARENTLY TAKEN THE DSCNT CLRNC TO FL240 MEANT FOR ACR Z. HE SAID THEY WERE STILL DISCUSSING THE ISSUE AND SUGGESTED WE CALL BACK. WE THEN FLEW TO CHICAGO AND CALLED THE SAME NUMBER. WE WERE EVENTUALLY PUT IN TOUCH WITH A SUPVR WHO HAD REVIEWED IT. OUR CONVERSATION WITH THE ZLA SUPVR WAS BOTH POSITIVE AND ENLIGHTENING. HE DID STATE THAT A PLTDEV RPT HAD BEEN FILED. HE STATED THAT THE CLRNC TO FL240 HAD BEEN GIVEN TO ACR XXY; AND WE (ACR XX) HAD ANSWERED AND DSNDED IN ERROR; CREATING THE CONFLICT WITH ACR Z. WE ACKNOWLEDGED THAT THE TAPE ALSO CLRLY RECORDED OUR READBACK OF ACR XX CLRED TO FL240. I WOULD LIKE TO LIST SOME DETAILS; FACTS; OPINIONS; AND RECOMMENDATIONS. ACCORDING TO THE TAPE; THE CTLR STATED ACR XXY AND WE THOUGHT IT WAS FOR US. WE DID READ BACK THE CLRNC CLRLY USING OUR CALL SIGN (ACR XX) NOT A 'ROGER;' NOT HALF A READBACK; BUT CALL SIGN AND ALT. WE HAD ASKED FOR A DSCNT CLRNC. WE THOUGHT THE CTLR WAS COMPLYING WITH OUR REQUEST. BEFORE WE REACHED FL270 WE THOUGHT WE HAD RECEIVED FURTHER CLRNC TO FL240. THE FO READ THE CLRNC BACK CLRLY AND CORRECTLY. SHE WAS THE ONLY FEMALE SPEAKING ON THE RADIO AT THAT TIME; GIVING HER A DISTINCTIVE IDENT. IT WAS ONLY 20 OR 30 SECONDS MAX BTWN OUR ACKNOWLEDGEMENT OF THE DSCNT TO FL270 AND FL240. MAYBE LESS TIME. ACR XXY HAD NOT EVEN CHKED ON THE FREQ UNTIL 2 OR 3 MINS LATER. THERE WERE NO OTHER XMISSIONS BLOCKING OR COVERING OURS -- THE FREQ WAS RELATIVELY QUIET. THE CRESO 3 STATES; EXPECT TO CROSS JOKUR AT FL240. MY EXPERIENCE HAS BEEN THAT WE ALWAYS GET A CLRNC TO FL240. THE CTLR KEPT BROADCASTING IN THE BLIND FOR ACR XXY; EVEN THOUGH HE HAD NOT CHKED IN OR ESTABLISHED RADIO CONTACT. WE WERE WELL RESTED. IT WAS IN THE MIDDLE OF THE DAY. IT WAS OUR FIRST LEG FLYING AFTER DEADHEADING FROM PHX TO SAN. WE HAD LUNCH JUST BEFORE DEP. THE FO AND MYSELF HAVE FLOWN TOGETHER SEVERAL TIMES. I CONSIDER HER AN EXCELLENT PLT. CRM: THE COCKPIT WAS STERILE. THERE WERE NO JUMP SEAT RIDERS. THE FLT ATTENDANTS WERE NOT UP GETTING US A DRINK. BOTH PLTS WERE IN THEIR SEATS. NO ONE WAS EATING OR TALKING ON THE PA OR USING #2 RADIO. WE BOTH 'THOUGHT' WE HEARD 'ACR XX; DSND TO FL240.' WE CONFIRMED IT WITH EACH OTHER; USED COMPANY PROCS FOR SETTING AND VERIFYING THE ALT. WE READ IT BACK. ATC: WE WERE TOLD BY THE SUPVR THAT THERE WERE 2 CTLRS LISTENING. THEY BOTH MISSED OUR READBACK. WE DSNDED OVER 2000 FT BEFORE (I ASSUME) SOME TYPE OF ALARM OR WARNING CAME ON TO ALERT THE CTLRS. THE CTR CTLR WAS CONTINUOUSLY BROADCASTING IN THE BLIND TO ACR XXY; AN ACFT WITH A VERY SIMILAR CALL SIGN. IF THEY HAD BEEN ON THE AIR; THAT WOULD HAVE BEEN 2 MORE PEOPLE LISTENING; AND THEY MAY HAVE RESPONDED TO THEIR CLRNC AND POINTED OUT THAT WE HAD TAKEN THEIR CALL. WE NEVER GOT A TCASII ALERT. APPARENTLY; WE WERE 2-3 MI AHEAD OF THE ACR Z FLT. THEY WERE NWBOUND; WE WERE NEBOUND. WE WERE DSNDING AND THEY WERE CLBING. I AM SURE OUR SPD WAS FASTER THAN THEIRS AND WE WERE INCREASING THE DISTANCE BTWN OURSELVES. RECOMMENDATIONS: 1) OBVIOUSLY WE HAVE TO LISTEN BETTER AND RESPOND ONLY TO OUR CALL SIGN. THE COMPANY SHOULD ATTEMPT TO SEGREGATE SIMILAR FLT NUMBERS BY TIME OR LOCATION. 2) ATC SHOULD PAY ATTN TO READBACKS FOR VERIFICATION. 3) ATC SHOULD NOT BROADCAST CLRNCS IN THE BLIND TO ACFT NOT ON THE FREQ. 4) MANY TIMES A COURTESY (I ASSUME) CALL IS MADE BY ATC TO TELL US A SIMILAR CALL SIGN IS OR WILL BE ON THE FREQ. THIS SHOULD BE MANDATORY. 5) COM IS A 2-WAY PROCESS. ALL PARTIES MUST LISTEN TO WHAT IS SAID; NOT WHAT THEY EXPECT TO HEAR. 6) WAS CTLR TRAINING GOING ON? SUPPLEMENTAL INFO FROM ACN 453713: I READ BACK 'ACR XX; DSND TO FL240.' AT THAT TIME WE WERE DSNDING THROUGH FL278 AND THIS SEEMED LIKE A REASONABLE TIME FOR ATC TO CONTINUE OUR DSCNT. WHEN WE FIRST HEARD THE ATC XMISSION; I LOOKED TO THE CAPT TO CONFIRM/ENSURE HE WANTED TO CONTINUE THE DSCNT TO FL240. HE NODDED; ACKNOWLEDGING HE HEARD THE CLRNC AND WANTED TO CONTINUE DOWN. HE ALSO DIALED IN 24000 ON THE FMC; POINTED TO IT; AND I VERBALLY CONFIRMED 240. WE CONTACTED THE ZLA SUPVR. HE ADVISED THAT APPARENTLY WE HAD RESPONDED TO A CLRNC FOR ACR XXY TO DSND TO FL240; WHICH CREATED A CONFLICT WITH ACR Z; WHERE; ACCORDING TO THE TAPES; SEPARATION WAS LOST. ADDITIONALLY; HE ACKNOWLEDGED THAT THE TAPE CLRLY RECORDED OUR READBACK OF 'ACR XX DSND TO FL240;' AND THAT THERE WERE 2 CTLRS ON THE SCOPE WHO MISSED MY READBACK AS WELL AS OUR TARGET LEAVING OUR APPARENT ASSIGNED ALT OF FL270. THE SUPVR ACKNOWLEDGED THAT; WHEN HE LISTENED TO THE TAPE; IT WAS VERY OBVIOUS THAT IT WAS THE SAME FEMALE VOICE THAT READ BACK THE 2 CLRNCS; ONE ISSUED RIGHT AFTER THE OTHER; WITHIN 10-15 SECONDS OF EACH OTHER. ACCORDING TO THE SUPVR; HE MENTIONED THAT THE CTLR WORKING OUR FLT WAS VERY FOCUSED ON ACR XXY AND TRYING TO GET THEM TO DSND. APPARENTLY BOTH CTLRS MISSED OUR TARGET FLASHING AS WE CONTINUED TO DSND THROUGH FL270 AND DSNDED ALL THE WAY DOWN TO FL245-FL246 WHICH IS ABOUT 2500 FT (APPROX 1 1/2 - 2 MINS OF DSCNT TIME). ON THE CRESO 3 ARR INTO LAS; IT STATES TO EXPECT FL240 XING JOKUR INTXN. THIS FURTHER ADDED TO OUR ANTICIPATION OF BEING DSNDED TO FL240. IN OUR 'BIG PICTURE' IT MADE PERFECT SENSE TO BE ISSUED A DSCNT CLRNC PRIOR TO LEVELING OFF SO IT IS A SMOOTHER DSCNT PROFILE AND BECAUSE JOKUR WAS COMING UP AND WE WERE PLANNING TO BE AT FL240 BY THEN. ACR XXY HAD NOT CHKED ON THE FREQ WHEN THE CLRNC TO FL240 WAS ISSUED. IF THEY HAD CHKED ON; WE WOULD HAVE BEEN AWARE OF A SIMILAR SOUNDING ACFT ON FREQ.