CTLR USE OF WRONG CALL SIGN SET UP POSSIBLE CONFLICT OF TWO IFR ACFT. OPERATIONAL DEVIATION.

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THIS OCCURRENCE TOOK PLACE BTWN 2 ACFT WITH SIMILAR CALL SIGNS. WE WERE IN THE AREA N OF SEA ISLE VOR WHEN ZDC INSTRUCTED OUR FLT TO DSND FROM 12000 TO 11000' AND TO EXPEDITE OUR DSCNT DUE TO TFC. WE ACKNOWLEDGED WITH FULL CALL SIGN AND FLT # READ BACK. WITH NO RESPONSE THAT THE CLRNC MIGHT HAVE BEEN FOR ANOTHER ACFT; WE STARTED OUR DSCNT EXPEDITIOUSLY AS REQUESTED. WHEN LEAVING 11600'; I HEARD A SIMILAR ACFT CALL SIGN ALSO RESPOND TO THE SAME CLRNC. I THEN ASKED CENTER TO VERIFY OUR CLRNC TO 11000' AND GOT NO RESPONSE FROM CENTER. I TRIED AGAIN TO VERIFY OUR CLRNC AND WAS CUT OUT BY ANOTHER ACFT. BY THIS TIME WE WERE AT 11400' WHEN ZDC CALLED OUR CALL SIGN AND TOLD US THAT WE WERE ASSIGNED 12000'. THE CTLR THEN GAVE US A DSCNT TO 10000'. AFTER LNDG IN PHL I CALLED CENTER TO CHK ON THE DEVIATION. THE SUPVR FOR THAT SHIFT LISTENED TO THE TAPE AND VERIFIED THAT MY RADIO XMISSIONS WERE RECEIVED LOUD AND CLEAR BUT THE CTLR DIDN'T RESPOND THAT WE HAD ACCEPTED A CLRNC FOR ANOTHER ACFT WITH SIMILAR CALL SIGN. SUPPLEMENTAL INFO FROM ACN 90545: WHEN NO LESS THAN 8 PEOPLE WANT TO TALK AT ONCE AND THE CTLR IS RUNNING 2 FREQS AND MANY LANDLINES; THERE IS A LOT OF ROOM FOR ERROR. WE NEED MORE QUALIFIED AIR TFC CTLRS! THE FREQS AND CTLRS ARE BOTH VERY OVERLOADED FOR SAFE FLT OPS.