CLRNC REPLY TO WRONG CALL SIGN RESULTS IN ALTDEV ALT EXCURSION.

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FLYING TO LAX FROM SAN. SECOND DAY ON LINE AFTER COMPLETING CAPT INITIAL OPERATING EXPERIENCE. THIRD FLT SAN-LAX IN 2 DAYS. (AFTER MANY (18-24) MONTHS SINCE FLYING THE RTE AS FO). INBOUND TO SEAL BEACH. CHKLISTS IN PROGRESS. FO RETURNS FROM BRIEFING PAX AND SAYS WHAT ARE YOU DOING; WE'RE SUPPOSED TO MAINTAIN 7000 FT. I SAY; WE WERE CLRED TO 4000 FT. I HAVE A DISTINCT MEMORY OF ACKNOWLEDGING THAT CLRNC. FO SAYS WE WERE CLRED TO 7000 FT; I AM SURE FO WAS OFFLINE WHEN 4000 FT CLRNC HAPPENED. LEVELED OFF AT 6500 FT; VERIFIED WITH ATC; THEN CLBED BACK TO 7000 FT. ALT DID NOT CAPTURE (ALT ARMED REPUSHED; BUT ALT ALERTER NOT RESET TO 7000 FT). ACFT CONTINUED CLB AND DID NOT EFFICIENTLY LEVEL OFF WHEN ALT HOLD BUTTON PUSHED. TOOK MANUAL CTL AT 7300 FT AND RETURNED TO 7000 FT. NO APPARENT TFC CONFLICT -- ATC NOT EXCITED. (BUT SUBSEQUENT VECTORS AND CLRNCS NOT AS CLOSELY ADHERED TO BECAUSE OF THE DISTR). ALL OK BY THE TIME TURNING TO FINAL. WHAT HAPPENED? I DON'T KNOW. POSSIBILITIES: I HEARD OR MISREAD CLRNC. BUT 7000 FT AND 4000 FT DON'T SOUND AT ALL ALIKE. I REMEMBER THINKING 4000 SEEMS A LITTLE LOW; BUT MAYBE THIS ISN'T OUT OF THE ORDINARY FOR OUR POS; WE WILL BE DSNDING TO LAND. (IN RETROSPECT; MY FO POINTED OUT THE FLOOR OF THE TCA IN THAT AREA IS ABOVE 4000; WHICH I DIDN'T KNOW. THE FLOOR IS AT 6000 FT). I FOLLOWED A CLRNC FOR ANOTHER AIRPLANE (BUT IF I RESPONDED; IT WAS NOT CAUGHT. DID I MAKE UP THE MEMORY OF RESPONDING?) I MADE UP THE 4000. (ALSO HARD TO BELIEVE). I RECEIVED A BOGUS CLRNC. NOTAMS SHOW THAT AS A POSSIBILITY IN THAT AREA. (THIS IS WHAT I SUSPECT HAPPENED). CONTRIBUTING FACTORS: COCKPIT DISTRS; INEXPERIENCE IN ACFT AND INEXPERIENCE (LACK OF RECENT) ON THAT RTE; ESPECIALLY WITH RESPECT TO THAT SPECIFIC AIRPLANE.