B727 CREW DSNDS WITHOUT CLRNC AFTER REPLYING TO WRONG CALL SIGN IN ZAU AIRSPACE.

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APCHING PIA WE REQUESTED A DSCNT. WE RECEIVED AND READ BACK A CLRNC TO 11000 FT. DURING DSCNT WE NOTICED WE COULD NO LONGER HEAR XMISSIONS FROM ZAU. WE ASKED ANOTHER FLT ON FREQ TO RELAY A HDOF; WHICH WAS SUCCESSFUL. UPON CHKING IN; DSNDED TO 11000 FT; THE CTLR ADVISED US WE HAD NOT BEEN CLRED TO DSND; AND HAD ACCEPTED ANOTHER FLT'S CLRNC (EVEN THOUGH IT WAS A RESPONSE TO OUR REQUEST). THE CTLR STATED THERE HAD BEEN NO CONFLICT (LUCKY!) AND INDICATED THERE WAS NO BIG PROB ABOUT IT FROM THEIR END. WE FEEL THE CTLR MAY HAVE THOUGHT HE WAS TALKING TO ANOTHER FLT; BUT ARE NEAR CERTAIN HE USED OUR CALL SIGN. WE DID READ BACK THE CLRNC WITH OUR CALL SIGN. WE FEEL IT WAS AN HONEST; WORKLOAD-RELATED MISTAKE THAT BOTH HE AND WE DID NOT CATCH; AND ARE GLAD IT RESULTED IN NO TFC CONFLICT. SUPPLEMENTAL INFO FROM ACN 419301: I REQUESTED A DSCNT FROM ZAU. CTR RESPONDED 'ABC XXX SWITCH TO 132.8.' SHE WAS VERY BUSY AND MADE NO MENTION OF OUR DSCNT REQUEST. I CHKED IN ON THE NEW FREQ AND MADE ANOTHER REQUEST FOR LOWER IN THE SAME XMISSION. ZAU RESPONDED 'ABC DSND TO ONE ONE THOUSAND.' I READ BACK OUR FULL CALL SIGN WITH THE NEW ALT '...ROGER; ABC XXX LEAVING TWO ZERO ZERO FOR ONE ONE THOUSAND.'