FLC OF LGT TOOK CLRNC MEANT FOR ANOTHER ACFT. DSNDING; HAD TCASII TA.

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AFTER COMING ON TO BUSY E COAST FREQ; WE COPIED: XX CROSS HOLEY AT 11000 PHILLY ALTIMETER 2980. AS CAPT/CHK AIRMAN I READ BACK THE CLRNC AND IN FULL USING XX SIMULTANEOUSLY RESETTING ALT ON FD PANEL. I WENT TO FMC TO SET VNAV XING HOLEY (?) NOT IN FMC. I CHKED APCH CHART NO HOLEY (?) SOMETHING WRONG. MEANTIME FO HAD INITIATED AN IDLE DSCNT (3500 FPM). A TA...I ACKNOWLEDGED. TCASII ONCOMING TFC. SOMETHING IS WRONG. I TOOK ACFT FROM STUDENT FO. 'I'VE GOT IT.' WHILE TRYING TO CALL CENTER I TOOK AUTOPLT OFF; AUTOTHROTTLES OFF; AND STARTED LEVELING ACFT. PWR UP AND CLBING BACK TO ALT. MORE COMS FROM CENTER. WE WERE ONLY HEARING CALL SIGN NUMBERS; NOT COMPANY IDENT. YYY CALLED TO CONFIRM THEIR CLRNC. NOW THE PROBLEM WAS SOLVED. I HAD READ BACK THE CLRNC INTENDED FOR YYY. WHY DIDN'T SOMEONE TELL ME WE HAD 2 ACFT ON FREQ WITH SAME CALL SIGN. WHY DIDN'T CTLR REPEAT CALL SIGN OF COMPANY? I COMPLAINED TO CTLR. MORE COM PROBLEMS. ZZZ COMPLAINED. CTLR WAS REPLACED. AFTER DISCUSSING WITH MY SUPVR I CALLED TCASII REPRESENTATIVE AND COMPANY MGR FOR ATC COM AND MGR HANDLING INDUSTRY WIDE SIMILAR CALL SIGNS. I HAVE SINCE FOUND OUT FROM TRANSCRIPTS THAT ACFT DEV OF 2.75 NM AND 400 FT.