B737-500 CREW TOOK A CLB CLRNC FOR A COMPANY ACFT WITH A SIMILAR CALL SIGN.

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MISUNDERSTOOD ALT CLRNC -- CALL SIGN DISCREPANCY. I WAS CAPT ON THE FLT. WE WERE FILED AT FL280 AND HAD BEEN INFORMED BY ZAB THAT FL220 WOULD BE OUR FINAL ALT. SHORTLY AFTERWARD; APPROX 40 NM N OF DRK VOR; WE WERE SWITCHED OVER TO ZLA (124.85). VERY SHORTLY AFTERWARD MY FO AND I BELIEVED WE HEARD CTR INSTRUCT COMPANY ABCD TO CLB AND MAINTAIN FL230; WHICH MY FO ACKNOWLEDGED ON THE RADIO AND I CLBED TO FL230. THOUGH WE HAD BEEN TOLD FL220 WOULD BE OUR FINAL; IT IS NOT UNCOMMON TO CHANGE AN ALT LATER FOR XING TFC OR OTHER REASONS. APPROX 2-3 MINS LATER; CTR ASKED US TO VERIFY LEVEL AT FL220. I REPLIED THAT WE HAD BEEN CLBED TO FL230. THE CTLR THEN REPLIED THAT THAT WAS FOR COMPANY ABC. I THEN DSNDED TO FL220 AND NOTHING MORE WAS SAID ABOUT THIS BY US OR ZLA. BOTH MY FO AND I WERE WELL RESTED AND THE WORKLOAD WAS NOT OVERWHELMING. DURING THIS TIME THE FO WAS VERIFYING THE LYNSY ARR IN THE FMC AND GETTING LAS ATIS. WE BOTH BELIEVED WE HEARD ABCD RATHER THAN ABCE ON THE RADIO. FO DID PLAINLY ACKNOWLEDGE WITH COMPANY ABCD. IF ABCE WAS ON THE SAME FREQ THEY DIDN'T CHALLENGE THIS INSTRUCTION. THE CTLR DID NOT MAKE US AWARE OF A COMPANY ABCE ON THE FREQ AS IS NORMAL PRACTICE. THIS OCCURRENCE ILLUSTRATES HOW IMPORTANT IT IS FOR ALL OF US TO MAINTAIN VIGILANCE AND QUESTION ANY INSTRUCTIONS THAT MIGHT NOT SOUND QUITE RIGHT. THE FACT THAT WE NOW HAVE FLTS OPERATING IN THE SAME AREAS AT THE SAME TIME WITH VERY SIMILAR CALL SIGNS/FLT NUMBERS UNDERSCORES THE NECESSITY FOR ALL OF US TO BE EXTRA CAREFUL. THIS OCCURRENCE REITERATES TO ME THAT IN THE FUTURE IF I HAVE ANY DOUBT OR QUESTION ABOUT AN INSTRUCTION; I WILL VERIFY OR CLARIFY BEFORE I TAKE ACTION.