SIMILAR SOUNDING CALL SIGNS GIVE THE FLC'S OF 2 CPR ACFT AND ATC A DIFFICULT TIME IN REPLIES TO WRONG CALL SIGNS ENRTE ON FREQ WITH ZNY AND ZOB.

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2 AIRPLANES (SAME COMPANY; SIMILAR CALL SIGNS) DEPARTED APPROX SAME TIME FOR THE SAME DEST. ONE FROM EWR AND ONE FROM TEB. BOTH UTILIZED THE SAME NEW YORK DEP; ZNY FREQ AND ZOB FREQS. ON NEW YORK DEP; THE EWR AIRPLANE ANSWERED A XMISSION MEANT FOR US. LATER; THEY ANSWERED ANOTHER XMISSION MEANT FOR US: 'CLB AND MAINTAIN 12000 FT.' BEFORE WE COULD ANSWER; THE EWR AIRPLANE REPLIED THAT THEY WOULD CLB AND MAINTAIN 12000 FT. WE STAYED AT OUR ALT UNTIL THIS WAS SORTED OUT. BECAUSE 2 APCH/DEP XMISSIONS MEANT FOR OUR AIRPLANE WERE MISTAKENLY TAKEN BY OUR COMPANY PLANE; WE FELT WE WERE HIGHLY AWARE OF LISTENING FOR OUR CORRECT CALL SIGN. WHEN CRUISING AT FL280; CTR ASKED OUR AIRPLANE IF WE COULD ACCEPT FL350. WE CONFIRMED THAT WE COULD. CTR THEN STATED FOR OUR PLANE TO CLB AND MAINTAIN FL330. WE CONFIRMED THIS CLRNC AND STARTED THE CLB TO FL330. AFTER PASSING FL290; CTR ASKED US TO 'VERIFY MAINTAINING FL280; YOUR MODE C INDICATES HIGH.' CTR THEN CAME BACK TO STATE; 'MAINTAIN FL280 AND TURN L TO HDG 250 DEGS.' WE ACKNOWLEDGED THAT WE WOULD MAINTAIN FL280 AND TURN TO A 250 DEG HDG. BOTH PLTS ON OUR AIRPLANE HEARD THE CLRNC FOR OUR CALL SIGN TO CLB TO FL330. AT OUR DEST; WE TALKED WITH THE CAPT OF OUR COMPANY AIRPLANE THAT LEFT EWR. HE CONFIRMED THAT HE HEARD CTR CLR US/OUR CALL SIGN TO FL330 -- BEFORE THEY ASKED US TO VERIFY MAINTAINING FL280. LATER; OUR COMPANY AIRPLANE THAT DEPARTED EWR ANSWERED ANOTHER XMISSION MEANT FOR US; ON ZOB. THIS INCIDENT PROVED TO BE AN INVALUABLE LESSON OF THE IMPORTANCE TO NOT ONLY KEEP A 100% CONCENTRATED LISTENING WATCH WITH CTLRS; BUT TO ALSO BE 100% SURE THAT THE CLRNC YOU ACCEPT IS FOR YOUR AIRPLANE. THIS BECOMES EVEN MORE DIFFICULT WHEN AIRPLANES WITH SIMILAR CALL SIGNS ARE ON THE SAME FREQ.