CTLR CONFUSED SIMILAR SOUNDING CALL SIGNS.

Narrative

AT 15000 FT ZDC TOLD ACR ABCD TO DSND TO 11000 FT. WE ANSWERED AND STARTED DOWN. ABOUT 14000 FT THE CTLR CAME BACK WITH 'ACR ABCD SAY YOUR ALT.' WE ANSWERED AND HE SAID 'THAT WAS FOR ACR ACCD.' WE ASKED IF HE WANTED US TO GO BACK TO 15000 FT AND HE SAID 'NO JUST STOP AT 13000 FT.' WE DID AND HE POINTED OUT SOME TFC BUT IT WAS FAR AWAY AND I DON'T BELIEVE A FACTOR IN THIS SIT. WE HEARD NO ACCD ON OUR FREQ UNTIL LATER ON. I BELIEVE THE CTLR WAS TALKING ON 2 FREQS AND DID NOT HEAR OUR RESPONSE; THAT IS ASSUMING WE ANSWERED ACCD CALL AND I DO NOT BELIEVE WE DID. MY FO AND MYSELF HEARD 'ACR ABCD DSND TO 11000 FT.' OUR COMPANY NEEDS TO KEEP THE CALL SIGNS DIFFERENT ON FLTS THAT ARE FLYING AT THE SAME TIMES. ABCD AND ACCD ARE TOO SIMILAR AND THE CTLR NEEDS TO TELL US HE IS TALKING TO ANOTHER ACFT OF A SIMILAR CALL SIGN (THEY USUALLY DO). WE HEARD NO FEEDBACK AS IF WE SPOKE OVER TOP OF SOMEONE. OUR COMPANY USUALLY KEEPS THE CALL SIGNS DIFFERENT BUT; NOT IN THIS CASE. I WILL INFORM THEM OF THIS PROB. I DO NOT THINK WE WILL BE VIOLATED ON THIS; IT DID NOT APPEAR TO PRESENT A BIG PROB TO THE CTLR; BUT WHO KNOWS?