FLC ACCEPTED DSCNT CLRNC INTENDED FOR ANOTHER ACFT.

Narrative

FLT FROM SFO TO BUR. ABOUT 10 MINS PRIOR TO PLANNED DSCNT ON THE FIM.FERN 5 ARR; I HAD TURNED DOWN ATC VOLUME TO MAKE A PREDSCNT PA (MORE LENGTHY THAN NORMAL BECAUSE WE EXPLAIN OUR SHORT FIELD LNDG PROCS AT BUR). WHILE THIS WAS OCCURRING; I HEARD THE CAPT RESPOND TO ATC 'OUT OF FL290 FOR FL240.' I SET FL240 IN OUR ALT WINDOW (AS PER SOP) BUT I DID NOT ACTUALLY HEAR ATC'S DIRECTIVE. 1 CTLR AT ZLA WAS CTLING SEVERAL SECTORS ON DIFFERENT FREQS. (WE HEARD SEVERAL OF HIS CALLS ON SIMULCAST; BUT NO ACFT RESPONSE.) WHILE WE THOUGHT IT WAS EARLY TO START THE DSCNT; AND WE USUALLY WERE GIVEN PLT'S DISCRETION; IT WAS NOT UNKNOWN TO DSND EARLY FOR FUTURE CONFLICTS; AND CTR DID NOT COUNTERMAND OUR CALL THAT WE WERE DSNDING. ABOUT 6-7 MINS LATER (WE HAD BEEN AT FL240 1 MIN OR SO) ZLA GAVE US 'PLT'S DISCRETION TO FL240.' WHEN I SAID WE WERE ALREADY DIRECTED TO FL240 AND WERE THERE; CTLR SAID WE HAD NOT BEEN CLRED EARLIER. I SUGGESTED HE REVIEW HIS TAPES. LATER; IN SOCAL APCH AIRSPACE; ANOTHER ACFT WITH A SIMILAR CALL SIGN CAME ON FREQ; BUT ARRIVED FROM THE NNE. WE HAD NOT HEARD THIS ACFT CALL SIGN WHILE ON ZLA FREQ EARLIER; BUT THE CTLR MAY HAVE BEEN CTLING HIM ON ANOTHER FREQ IN ANOTHER SECTOR; AND DIRECTED HIM TO FL240. OUR RESPONSE MAY HAVE MIRRORED THE OTHER ACFT'S CALL ON THE OTHER FREQ; SO THE CTLR DID NOT HEAR OUR CALL. BY THE WAY; WE WERE NOT ADVISED OF A SIMILAR CALL SIGN BY EITHER ZLA CTLR OR SOCAL APCH CTLR; BUT THE OTHER ACFT WAS ADVISED OF US BY SOCAL APCH. NO TFC CONFLICT RESULTED FROM THIS CONFUSION AND OUR EARLY DSCNT TO FL240.