DC8-71 CREW HAD THE CTLR CONFUSE FLTS WITH SIMILAR CALL SIGNS.

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WHILE WE WERE AT CRUISE AT FL220; ZID ON 128.07 CALLED FOR US TO DSND TO FL210. THE FO ACKNOWLEDGED AND I DSNDED THE ACFT. AS WE APCHED LEVELOFF; THE CTLR CALLED US AGAIN AND ASKED US WHY WE WERE DSNDING. THE FO RESPONDED THAT 'THAT IS WHAT THE CTLR HAD CLRED US TO DO.' THE CTLR ANSWERED THAT WAS MEANT FOR CALL SIGN Y; NOT CALL SIGN X; AND FOR US TO GO BACK TO FL220. SO I DID. THEN ABOUT 1 MIN LATER; THE CTLR CALLED CALL SIGN Y AND TOLD HIM TO DSND TO FL210! OBVIOUSLY; CALL SIGN Y HAD NOT HEARD ANY CLRNC TO DSND. WE COULD HEAR THE CTLR MAKE THAT CALL; BUT THE CALL SIGN Y ACFT ANSWERED ON A DIFFERENT FREQ. IT SEEMED THAT ALL OTHER ACFT WERE ON FREQS OTHER THAN THE ONE WE WERE ASSIGNED. LATER; AS HE WAS ABOUT TO HAND US OFF; THE CTLR MORE OR LESS APOLOGIZED FOR THE COMS MIX-UP; MENTIONING SIMILAR CALL SIGNS. SUPPLEMENTAL INFO FROM ACN 566730: I AM QUITE SURE THE CTLR CLRED CALL SIGN X TO MAINTAIN FL210 WHEN HE OBVIOUSLY MEANT TO CLR CALL SIGN Y. UNFORTUNATELY. THE CTLR DID NOT CATCH THE MISTAKE IN CLRNCS AFTER WE READ BACK THE CLRNC WE HEARD HIM GIVE TO US.