ACR MLG ALT DEVIATION EXCURSION FROM CLRNC ALT IN RESPONSE TO WRONG CALL SIGN.

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THIS IS THE SECOND ALT DEVIATION CLRNC MISCOM THAT I HAVE BEEN INVOLVED WITH IN THIS SECTOR OF OAK CENTER. IT IS A VERY BUSY AREA WITH DEPS FROM SFO OVER LINDEN AND SFO ARRS OVER MODESTO. WE WERE FLT UXXY; SBND TO SAN DIEGO. ANOTHER FLT; UXYX WAS ALSO ON THE SAME FREQ 126.85. WE WERE INITIALLY CLRED TO FL190; THEN TO FL210. BOTH ALT CLRNCS WERE CLRLY READ BACK TO CTLR. AS WE LEVELED AT FL210; OAK CENTER ASKED 'WHAT WAS YOUR LAST CLRED ALT?' WE RESPONDED 'FL210.' OAK CENTER STATED THAT THE CLRNC TO FL210 WAS FOR COMPANY FLT UXYX NOT UXXY. NO INCIDENT OR ENCOUNTER WITH ANY OTHER ACFT. IT APPEARS THAT THE CTLRS OF THIS OAK SECTOR ARE SO BUSY THAT THEY MISS INCORRECT CLRNCS; EVEN WHEN THEY ARE READ BACK. A NEAR MISS OR MID-AIR MAY BE JUST A MATTER OF TIME IN THIS AREA. I DO NOT BELIEVE THAT THE FLT NUMBERS WERE TOO SIMILAR TO CONTRIBUTE TO THIS ERROR. THE SECTOR IS OVERLOADED WITH CIVILIAN AND MIL TFC. SUPPLEMENTAL INFO FROM ACN 87347. PREVIOUS TO THIS; WE WERE UNAWARE OF COMPANY UXYX PRESENCE ON THE FREQ.