Narrative:

Misunderstood altitude clearance -- call sign discrepancy. I was captain on the flight. We were filed at FL280 and had been informed by ZAB that FL220 would be our final altitude. Shortly afterward, approximately 40 NM north of drk VOR, we were switched over to ZLA (124.85). Very shortly afterward my first officer and I believed we heard center instruct company abcd to climb and maintain FL230, which my first officer acknowledged on the radio and I climbed to FL230. Though we had been told FL220 would be our final, it is not uncommon to change an altitude later for crossing traffic or other reasons. Approximately 2-3 mins later, center asked us to verify level at FL220. I replied that we had been climbed to FL230. The controller then replied that that was for company abc. I then descended to FL220 and nothing more was said about this by us or ZLA. Both my first officer and I were well rested and the workload was not overwhelming. During this time the first officer was verifying the lynsy arrival in the FMC and getting las ATIS. We both believed we heard abcd rather than abce on the radio. First officer did plainly acknowledge with company abcd. If abce was on the same frequency they didn't challenge this instruction. The controller did not make us aware of a company abce on the frequency 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/flight 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.

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Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.