ZLA CTLR PROVIDING OJT EXPERIENCED OPERROR AT FL300 WHEN INCORRECT DESCENT ALT WAS MISSED; RESULTING IN CONFLICT.

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C500 ASSIGNED FL300. ACFT READ BACK CLRNC. DUE TO POOR RADIO; HEARD FL300; PLT ACTUALLY READ BACK FL200. ACFT TOLD TO STOP DSCNT WHEN CONFLICT ALERT ACTIVATED. SUPPLEMENTAL INFO FROM ACN 766796: I WAS ON A VECTOR DURING DSCNT FROM FL330. I THOUGHT THE CTLR HAD TOLD ME TO DSND AND MAINTAIN FL200. MY READBACK WAS THAT I WOULD DSND AND MAINTAIN FL200. UPON REACHING FL293; THE CTLR ASKED ME TO CONFIRM FL300. I LEVELED OFF AT FL292 AND RESPONDED THAT I HAD UNDERSTOOD I WAS TO DSND AND MAINTAIN FL200. THE CTLR ASKED ME TO MAINTAIN PRESENT ALT AND TURN R 15 OR 20 DEGS (I DON'T RECALL WHICH). AFTER A COUPLE MINS; THE CTLR ASKED ME TO MAINTAIN FL290; WHICH WAS LATER FOLLOWED BY LOWER ALT IN PREPARATION FOR ARR INTO VGT ARPT. THERE WAS NO INDICATION OF A TFC OR AIRSPACE CONFLICT OTHER THAN THE SHORT VECTOR FOR WHICH I DON'T REMEMBER A REASON BEING GIVEN. ALTHOUGH THERE WAS MINIMAL NOISE ON THE INTERCOM DUE TO THE COCKPIT MIKES BEING SELECTED TO THE 'OPEN MIKE;' I THINK THE CAUSE WAS SIMPLY MY MISUNDERSTANDING THE NEW ASSIGNED ALT. A CONTRIBUTING FACTOR WAS THAT THE CTLR DID NOT CORRECT ME WHEN I READ BACK THAT I WAS DSNDING TO FL200. TO PREVENT THIS FROM OCCURRING IN THE FUTURE; I PLAN TO KEEP THE INTERCOM IN THE 'CLOSED MIKE' POS UNLESS FLT CREW CONVERSATION IS NECESSARY. THIS WILL IMPROVE CLARITY OF ALL RADIO COMS.