B767-300 CAPTAIN REPORTS UNAUTHORIZED DESCENT DUE TO SIMILAR CALL SIGNS AND ARTCC CTLR WORKLOAD.

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OUR ACFT WAS LEVEL AT FL360; NEARING TOP OF DSCNT ONTO THE ARR INTO JFK. OUR NEXT ACTION FIX WAS TRAIT. OUR CALL SIGN WAS ACR X ABA; AND ACR X ACA AND ACR Y ABA WERE ALSO ON FREQ. ZBW WAS THE CTLING AGENCY. THE FREQ WAS VERY BUSY WITH ALMOST 'NON-STOP' CTLR DIRECTION AND SHORT AIRCREW RESPONSES. SEVERAL CREWS REQUESTED REPEATS ON THE CALLS; AS THEY WERE NOT SURE WHO THE CALL WAS FOR. WE HEARD A CALL FOR AN ACR X '(SOMETHING) A' TO DSND TO CROSS FIX TRAIT AT 240 AND SWITCH TO ANOTHER FREQ. TRAIT WAS LOCATED IN FRONT OF US; AT A PROPER DISTANCE TO BEGIN DSCNT. I DID NOT HEAR ANY OTHER ACFT ANSWER; AND SPECIFICALLY ASKED THE CTLR IF THE CALL WAS FOR ACR X A-B-A. HE CONFIRMED IT WAS. WE SWITCHED FREQS AND STARTED OUR DSCNT TO CROSS TRAIT. ON CHK-IN THE NEXT CTLR TOLD US TO RETURN TO OUR PREVIOUS FREQ. WHEN WE DID; THE BOSTON CTLR WAS ADMONISHING US FOR CHANGING FREQS; AND TOLD US TO STOP OUR DSCNT AT FL340. I REMINDED HIM THAT I HAD SPECIFICALLY ASKED IF THE CLRNC HAD BEEN FOR US; AND THAT HE HAD CONFIRMED. HE STATED THAT 'IT WAS NOT A PROB...JUST STOP OUR DSCNT AT FL340.' SEVERAL MORE TIMES DURING VECTORS ON DSCNT AND APCH; THE CTLR OR OTHER ACFT HAD DIFFICULTY WITH THE SIMILAR SOUNDING CALL SIGNS. THE LAST WHEN ACR Y ABA TOOK A VECTOR MEANT FOR US TO INTERCEPT FINAL. RECOMMENDATIONS: WHEN A PARTICIPANT IN THE RADIO COMS PROCESS GETS BUSY; IT IS ESSENTIAL TO 'MAKE TIME' OR SLOW DOWN TO COMMUNICATE EFFECTIVELY. POOR COM CAN BE MUCH WORSE THAN NO COM. WHERE THERE WERE SIMILAR SOUNDING CALL SIGNS; NOTIFICATION OF THAT SITUATION IS IMPORTANT; BUT TRY TO DISTINGUISH THE PARTIES BY ENUNCIATING MORE CLRLY; AND/OR SPELLING THE CALL SIGNS. LISTEN FOR THE CORRECT RESPONSE; AND DO NOT CONFIRM A CLRNC OR RESPONSE UNLESS YOU ARE CERTAIN THAT THE CORRECT COM WAS RECEIVED. I BELIEVE THAT IF THE CTLR HAD BEEN LESS PRESSURED; HE WOULD HAVE REALIZED THAT IT WAS NOT THE ACFT HE HAD INTENDED THAT WAS ASKING HIM TO CONFIRM THE CLRNC. THE CTLR WAS SPEAKING SO FAST THAT REPEATS WERE BEING ASKED FOR. INSTEAD OF ACR X ABA; ACR X A-B-A IS NOTICEABLY MORE EFFECTIVE IN GETTING THE CORRECT RECIPIENT. IF BOSTON; OR ANY CTLR IS SO BUSY THAT HE/SHE CANNOT COMMUNICATE EFFECTIVELY; THE SUPERVISION MUST EITHER SPLIT THE FREQ OR ADD A CTLR. IT IS NOT UNCOMMON TO HAVE SIMILAR SOUNDING CALL SIGNS WITH THE THOUSANDS OF FLTS AIRBORNE. THE SYS MUST BE ABLE TO ACCOMMODATE THEM EFFECTIVELY. AIRCREWS MUST BE AWARE OF THE CTLR'S LOAD SIT; AND MAKE EXTRA EFFORTS TO ENSURE THEY ARE RECEIVING AND RESPONDING TO THE CORRECT CLRNCS. WE THOUGHT WE HAD DONE THAT BY SPECIFICALLY ASKING THE CTLR TO CONFIRM THE DSCNT CLRNC AND FREQ CHANGE WAS FOR US.