ACR X RESPONDED TO ACR Y CLRNC UNAUTHORIZED DSCNT. PLTDEV. ACR X HAD LTSS FROM ACR Y. SYS ERROR.

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ACR X AND Y WERE INBOUND FLTS TO PHX. THE NIGHT TIME CONDITIONS WERE TSTMS IN AREA WITH ACFT DEVIATING THROUGHOUT THE SECTOR BECAUSE OF WX ON BOTH INBOUND AND OUTBOUND RTES. ACR Y WAS AT FL310 AND ACR X ALMOST DIRECTLY ABOVE AT FL330. BOTH ACFT NEEDED TO BE SPACED FOR DSCNT AND ARR TO PHX. ACCORDINGLY; THE RADAR CTLR ISSUED CLRNC FOR ACR Y TO TURN R TO A HDG OF 260 AND DSND TO 12000. THE READBACK OF THIS CLRNC (THE TAPES LATER SHOWED) WAS CORRECT EXCEPT THAT THE CALL SIGN AT THE END (NOT THE BEGINNING) WAS ACR X. ACR X DSNDED INTO CONFLICT WITH ACR Y AFTER TAKING ACR Y'S CLRNC. THE CTLRS (BOTH R AND D) WERE HELD AT FAULT BECAUSE THEY DID NOT CATCH THE 'ACR X' AT THE END OF THE READBACK. AS THE RADAR CTLR SAW ACR X DSND (NO RPT LEAVING THE ALT) HE ISSUED INSTRUCTIONS TO KEEP THE INCIDENT FROM TURNING INTO AN ACCIDENT. WHOEVER WAS TO BLAME; THIS POINTS UP THE NEED FOR PLTS AS WELL AS CTLRS TO LISTEN TO THE RADIO WITH CARE. 3 CREW MEMBERS IN THE ACR Y COCKPIT SHOULD HAVE HEARD ACR X ANSWER THEIR CLRNC. THEY SAID NOTHING. BOTH PLTS IN ACR X TOOK ACTION ON A CLRNC NOT DIRECTED TO THEM.