CL65 IS CONFUSED REGARDING ATC ASSIGNED ALT IN ZLA AIRSPACE.

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OUR ACFT HAD LEVELED AT FL290; AND WERE THEN ISSUED A CLB BY LAX CTR (125.72) TO FL310. ADMITTEDLY; I WAS PERPLEXED AS WE WERE FILED FOR FL290; BUT I ASSUMED HE WAS STEPPING US UP TO FL330 AS WE WERE EBOUND FROM LAX TO SLC. OUR ACFT WAS THEN HANDED OFF TO LAX CTR (132.62). WHILE CLBING THROUGH FL298; THE CTLR ASKED HOW LONG IT WOULD TAKE US TO BE AT FL290. I REPLIED 'ABOUT 30 SECONDS.' HE THEN ASKED HOW LONG IT WOULD TAKE US TO REACH FL330. I TOLD HIM ABOUT 5 MINS. HE REQUESTED THAT WE DSND AND MAINTAIN FL290. HE THEN STATED THAT FL310 WAS WRONG FOR THE DIRECTION OF FLT; AND THAT THE LAST CTLR DIDN'T KNOW WHERE WE GOT FL310 FROM. I TOLD HIM THAT THE LAST CTLR DID; IN FACT; ISSUE A CLB TO FL310; AND THAT WE WERE A BIT PERPLEXED AS WE WERE FILED TO FL290; BUT THAT I ASSUMED THAT HE WAS STEPPING US UP TO FL330. A FEW MOMENTS LATER; I TOLD THE CTLR THAT IF THE LAST CTLR STILL HAD A QUESTION; HAVE HIM MARK THE TAPE AND GO BACK AND LISTEN TO IT. THE CTLR MADE NO FURTHER COMMENT; NOR WAS THERE ANY FURTHER DISCUSSION PURSUANT TO OUR ATC GENERATED ALT MISASSIGNMENT. IT IS BECOMING ALL TOO COMMON AN OCCURRENCE TO SAY THE WRONG ACFT CALL SIGN WHEN ISSUING CLRNCS. ON AN AVERAGE 4-DAY TRIP; I HEAR ATC MAKE THIS MISTAKE AT LEAST 7-8 TIMES. PERHAPS MORE DISCONCERTING IS THE FACT THAT ATC CTLRS RARELY ACTUALLY LISTEN TO A CREW READBACK. I HEAR MANY MISCOMMUNICATIONS OCCUR; AND ATC RARELY RESPONDS WHEN AN INCORRECT READBACK IS XMITTED. NEEDLESS TO SAY; I WILL NO LONGER GIVE ATC THE BENEFIT OF THE DOUBT; AND WILL QUERY ANY QUESTIONABLE ALT ASSIGNMENT.