ZLA CTLR EXPERIENCED OPERROR AT FL286 WHEN READBACK FROM PLT WAS NOT CLEAR AND DSCNT WAS INITIATED. FREQ HAS UCR PENDING.

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WORKING R-53 WITH APPROX 6 ACFT ON FREQ. THE FREQ HAS AN UNSATISFACTORY CONDITION RPT (UCR) OUTSTANDING FOR POOR QUALITY. 10 MI IN-TRAIL TO PHX WITH 4 OF THE ACFT LNDG PHX AND ON VECTORS. ACR X #2 IN LINE AND ON A 145 VECTOR FOR SPACING. I ISSUED FL290 AND ACR X PLT READ BACK; 'TWO NINE ZERO; ACR X.' THE KING AIR WAS WBOUND AT FL280. I THEN ISSUED; 'ACR X FLY HDG 060; EXPECT DIRECT WOTRO IN ABOUT A MIN.' PLT RESPONDED; 'ZERO SIX ZERO; ACR X.' (IT WAS LATER DETERMINED THAT THE PLT READ BACK TWO SIX ZERO; BUT THE POOR RADIO QUALITY REQUIRED 3 REPLAYS TO DETERMINE THAT.) CONFLICT ALERT ACTIVATED AS ACR X'S MODE C INDICATED FL286. I CLRED ACR X TO MAINTAIN FL290 AND THE PLT STATED HE WAS CLRED TO FL260. CONTRIBUTORY TO THIS EVENT: POOR RADIO QUALITY. IF I HAD CLRLY HEARD THE PLT'S READBACK OF 'TWO SIX ZERO' INSTEAD OF 'ZERO SIX ZERO;' I COULD HAVE REISSUED THE HDG OF 060 DEGS TO ALLEVIATE THE PLT'S CONFUSION. POOR PHRASEOLOGY BY THE PLT. THE PLT'S READBACK OF 'TWO-SIX-ZERO' GAVE ME NO INDICATION THAT THE CLRNC WAS UNDERSTOOD TO BE AN ALT VERSUS A HDG. IF HE HAD SAID 'DSND TO 260' OR 'FL260;' I MAY HAVE HAD AN OPPORTUNITY TO CORRECT IT. AS IT WAS; THE 'TWO-SIX-ZERO' READBACK GAVE ME NO CHANCE AT ALL. I FIND THIS INCIDENT QUITE UNSETTLING. CTLRS ARE UP AGAINST IT NOW WITH EVER INCREASING TFC LEVELS AND REDUCED STAFFING LEVELS. ADD IN RADIOS THAT DON'T PERFORM AND PLTS THAT DON'T USE PROPER PHRASEOLOGY AND YOU CAN BET THAT IT'S JUST A MATTER OF TIME BEFORE WE SEE SOMETHING CATASTROPHIC.