ACR XXY HAS TCASII TA ON ACR ZXY XING TFC IN DSCNT.

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ZXY FROM EWR TO SFO. XXY FROM PIT TO SFO. ON MORE THAN ONE DAY; BOTH ACFT HAVE BEEN ON THE SAME FREQ; SAME ROUTING; EXPECTING THE SAME CLRNCS. IT HAS BEEN RPTED AS A SIMILAR CALL SIGN PROBLEM WITH NO RESOLUTION BECAUSE THE SCHEDULED 'BLOCK' TIMES ARE NOT IN CONFLICT. (?; FLTS SHOW BLOCK TIMES 2 MINS APART). WHILE WORKING THE LOW ALT CTR SECTOR; I TOOK A HDOF ON ZXY; SHOWED LEVEL AT FL240. XXY CHKED ON AND WAS ISSUED (TO ZXY) A DSCNT CROSS CEDES AT 110 AND 250 KTS. XXY HAD BEEN ISSUED FL280 FOR TFC FROM THE HIGH ALT SECTOR. ZXY WAS AT FL240; ON MY SCOPE; BUT NOT ON FREQ. XXY RPTED TCASII TFC. I ADVISED ZXY THAT I HAD NO TFC. AT THAT TIME ZXY CAME ON FREQ REQUESTING LOWER AND ADVISING THEY WERE TOO HIGH TO MEET THE EXPECTED RESTRICTION. I ASKED WHICH ACFT HAD THE TCASII ALERT. XXY RESPONDED AND I ASKED HIS ALT. XXY WAS PASSING THROUGH FL250 AND I STOPPED THE ACFT AT FL240. THE HIGH ALT SECTOR REALIZED WHAT HAD HAPPENED AND GAVE ME THE HDOF ON XXY; WHICH I WOULD HAVE WORKED SOON ANYWAY. XXY HAS APPARENTLY RESPONDED TO THE TCASII; LEVELED OFF IN THE DSCNT; AND AVOIDED TFC AT FL260 THAT HE HAD BEEN STOPPED FOR. ZXY DID NOT CHANGE FREQS. XXY TOOK ZXY'S FREQ CHANGE. XXY ACKNOWLEDGED ZXY'S CLRNC TO 110; I DID NOT RECOGNIZE CALL SIGN DIFFERENCE -- NOT AWARE THAT XXY WAS EVEN ON FREQ BECAUSE I HAD HDOF AND SHOULD HAVE BEEN TALKING TO ZXY. THIS WAS DURING A BUSY EVENING RUSH INBOUND TO SFO. THE CTLR AT HIGH ALT WAS A DEVELOPMENTAL WITH ONE RADAR CERTIFICATION. LOW ALT WAS COMBINED WITH A DEP SECTOR -- SINGLE CTLR. ZXY WAS A WDB AND XXY WAS AN MLG. IF 'HVY' WERE STILL REQUIRED; THIS MAY HAVE HELPED. TCASII ON XXY PREVENTED A POTENTIAL DISASTER. XXY HAD TAKEN ZXY'S FREQ CHANGE AT LEAST ONCE EARLIER IN FLT. THEY WERE ADVISED OF EACH OTHER AT LEAST ONCE. SOLUTION: WHEN CTLRS SAY THERE IS A CALL SIGN PROBLEM -- BELIEVE IT; AND CHANGE THEM! SUPPLEMENTAL INFO FROM ACN 214975. CHKED IN WITH OAK CTR AT FL310; FULL CALL SIGN OF XXY WAS USED IN ALL XMISSIONS. CLRNC TO CROSS MODESTO AT 250 KTS; 11000 FT WAS ISSUED. BOTH FO AND MYSELF BELIEVED IT TO BE FOR US. I ACKNOWLEDGED WITH FULL CALL SIGN AND PF STARTED DSCNT. PASSING THROUGH 26000 FT WE RECEIVED A TA ON 9 O'CLOCK TFC. I ASKED OAK CTR ABOUT IT; USING FULL CALL SIGN AND THEY RESPONDED THAT THEY DIDN'T SEE ANY TFC. WE WERE NOT TOLD OF ZXY HVY BEING ON FREQ CTLR NEVER CALLED ZXY HVY; ONLY ZXY. CTLR OBVIOUSLY DID NOT LISTEN TO OUR FULL CALL SIGN. IT'S POSSIBLE THAT WE SUBCONSCIOUSLY HEARD XXY INSTEAD OF ZXY; AS WE WERE EXPECTING DSCNT. SIMILAR CALL SIGNS ARE DANGEROUS. FURTHER INQUIRY SHOWED THAT CTR HAS THE CALL SIGN CONFLICT EVERY NIGHT AND YET DID NOT TAKE PRECAUTIONS.