B777 FLT CREW INADVERTENTLY TAKES ANOTHER SIMILAR ACFT'S CALL SIGN AND DSNDS ON THAT CLRNC. ATC CTLR RECOGNIZES THE MISTAKE AND ISSUES ANOTHER CLRNC.

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WE WERE ON A HDG W ON DOWNWIND FOR RWY 06L AT 8000 FT N OF THE LAX ARPT AND WERE GIVEN DSCNT TO 3500 FT. WE READ BACK SAME; SET MCP AND STARTED TO DSND AS PER SOPS. AT ABOUT 6000 FT ON OUR DSCNT; ATC INSTRUCTED US TO DSND TO 5000 FT. WE RESET MCP TO 5000 FT AND LEVELED OFF THERE. ONCE ON BASE LEG WE HEARD OTHER ACFT BEING DSNDED TO 3500 FT FOR A FIX ON FINAL WHICH WE WERE LATER GIVEN AND IT RAISED THE QUESTION WHETHER WE WERE IN ERROR EARLIER WHEN WE ACCEPTED THE FIRST 3500 FT DSCNT CLRNC. WHEN WE QUERIED ATC ABOUT SAME THEY REPLIED WE PROBABLY TOOK ANOTHER ACR'S DSCNT CLRNC AND REPLIED 'NO WORRIES.' I FEEL THAT WE WERE BEING WATCHED IN OUR APCH AND THERE WAS NEVER ANY CONCERN FOR TFC/TERRAIN CONFLICT. AFTER LISTENING TO THIS PARTICULAR CTLR FOR A FEW MINS IT OCCURRED TO ME THAT HE WAS NOT USING THE COMPANY NAME IN ANY OF HIS CLRNCS ONLY THE FLT NUMBER SO WE MUST HAVE HEARD THEIRS INSTEAD OF OURS OR POSSIBLY WE HEARD IT RIGHT AND HE GAVE THE WRONG NUMBER THE DSCNT CLRNC. I FEEL HIS 'TECHNIQUE' IS VERY INAPPROPRIATE FOR A CTLR ESPECIALLY IN A BUSY TFC ENVIRONMENT SUCH AS LAX. SUPPLEMENTAL INFO FROM ACN 692572: WE WERE UNAWARE OF ANOTHER FLT WITH A SIMILAR SOUNDING CALL SIGN. EITHER OUR ACCEPTANCE OF ANOTHER FLT'S CLRNC WAS IGNORED (NOT LIKELY) OR BOTH FLTS ANSWERED SIMULTANEOUSLY AND OUR RESPONSE WAS TOTALLY BLOCKED MAKING THE CTLR UNABLE TO CORRECT THE ERROR. THE CTLR'S INSTRUCTIONS WERE REPEATEDLY CLIPPED AT THE BEGINNING OF 50-60% OF HIS XMISSIONS. AIRLINE NAMES WERE MISSING FROM INSTRUCTIONS AND 4 DIGIT FLT NUMBERS WERE OFTEN NOW 3 DIGITS. THIS WAS EITHER DUE TO CTLR EQUIP MALFUNCTION; IMPROPER RADIO TECHNIQUE (HIGHLY UNLIKELY FROM ATC -- THIS IS NORMALLY A PLT ISSUE WHEN NOT USING A FULL CALL SIGN) OR; MOST LIKELY; THE CTLR'S MIKE BUTTON FINGER WAS OUT OF SYNC WITH HIS MOUTH. THIS INCIDENT SHOWS THAT EVEN WHILE OPERATING UNDER ALL SOP'S IN A DISCIPLINED COCKPIT; A TECHNOLOGY BREAKDOWN (2 ACFT SIMULTANEOUSLY XMITTING) CAN STILL LEAD TO ERRORS.