ALT EXCURSION; HDG TRACK DEV WHEN A B737-300 FLC ACCEPTS AND READS BACK A CLRNC INTENDED FOR ANOTHER COMPANY FLT ON APCH FREQ FOR SAN; CA.

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TOOK ANOTHER ACFT'S CLRNC. WE WERE ON A DOWNWIND VECTOR FOR A LOC RWY 27 APCH IN SAN; HDG 110 DEGS AT 6000 FT. WE HEARD A CLRNC THAT WE READ BACK THE CLRNC; AND THE FO TURNED AND DSNDED. BEFORE WE ROLLED OUT ON THE 180 DEG HDG; THE CTLR CALLED AND GAVE US A L TURN BACK TO 070 DEGS. I RESPONDED TO HIM AND ALSO SAID WE WERE AT 5000 FT AND ASKED IF HE WANTED US TO CLB BACK TO 6000 FT. HE SAID FOR US TO MAINTAIN 5000 FT; AND THAT THE EARLIER CLRNC WAS FOR ANOTHER ACFT. WE CONTINUED ON TO A NORMAL APCH AND LNDG. THE CAUSE WAS NO DOUBT OUR FAILURE TO VERIFY THE CLRNC ON A VERY BUSY FREQ. I ALSO BELIEVE THAT FATIGUE WAS A FACTOR. WE WERE ON THE LAST LEG OF THE DAY AFTER AN ACFT CHANGE AND A 2 HR GND TIME. I THINK CTLRS COULD HELP PREVENT SITS LIKE THIS BY TELLING US OUR SEQUENCE OR WHO WE ARE FOLLOWING. THIS ALWAYS HELPS BY GIVING YOU THE BIG PICTURE; HOWEVER; DUE TO THE WORKLOAD; THIS USUALLY ISN'T DONE OR POSSIBLE. SUPPLEMENTAL INFO FROM ACN 572408: APCH CTLR CAUGHT IT IMMEDIATELY AND TURNED US BACK ON ORIGINAL HDG. WE APOLOGIZED AND HE SAID 'NO PROBS; AS THERE IS A VERY SIMILAR CALL SIGN IN PATTERN AS WELL.' THERE WAS NO TA/RA OR A TFC CONFLICT.