CREW IN B737-300 INBOUND TO PIT TAKE DSCNT CLRNC MEANT FOR COMPANY WITH THE SAME LAST 2 NUMBERS OF THE FLT IDENT.

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ENRTE FROM ABE TO PIT AT 22000 FT GETTING NEAR PSB VOR; WE GOT A NEW RADIO FREQ OF 121.4; WHICH IS ZOB. I (FO) SWITCHED OVER AND CHKED IN WITH HIM. HE ACKNOWLEDGED ME AND THAT WAS ALL. (IT WAS SATURDAY AND THE FREQ WAS VERY QUIET.) ABOUT 2 MINS LATER I HEARD WHAT I THOUGHT WAS FOR US; ACR X; TO CROSS GRACE INTXN AT 10000 FT; 250 KTS. I ACKNOWLEDGED HIM WITH A READBACK INCLUDING CALL SIGN. THIS IS NORMAL AT THIS TIME; AT ABOUT 100 NM FROM GRACE INTXN AND WE ALREADY HAD IT (AT THIS POINT WE NEVER HEARD A BROADCAST FROM ACR Y). WE PROGRAMMED IN ON THE FMC. IT STARTED GETTING CHOPPY SO WE STARTED DOWN. AT ABOUT GOING THROUGH FL210 ATC SAID ACR X MAINTAIN FL200. SO WE STOPPED AT FL200. I ASKED HIM IF HE STILL WANTED GRACE INTXN AT 10000 FT/250 KTS. HE REPLIED WE WERE NOT GIVEN GRACE AT 10000 FT/250 KTS. I IMMEDIATELY QUESTIONED HIM SAYING WE WERE AND I READ IT BACK. HE SAID NO THAT WAS FOR ACR Y; AND STATED; NO PROB JUST MAINTAIN FL200. NO CONFLICT WAS CAUSED. PROB BEING: CALL SIGNS SO MUCH ALIKE NEITHER THE CTLR OR THE OTHER ACFT; ACR Y; CAUGHT THIS. WE WERE NOT INFORMED THAT THERE WAS AN ACR Y ON FREQ. THESE CALL SIGNS; ACR X AND ACR Y SOUND SO MUCH ALIKE; MUCH MORE THAN MOST. THEY SHOULD NOT BE USED IN THE SAME AIRSPACE. WHEN YOU SAY ACR X NOT ACR XX IT SOUNDS SO MUCH LIKE ACR Y THAT THE CAPT; MYSELF; THE OTHER ACFT; AND THE CTLR NEVER PICKED UP ON WRONG ACFT GETTING WRONG CLRNC.