CONFUSION BY ACR FLC ON WHICH CALL SIGN WAS USED BY DEP CTLR. ALTDEV ALT EXCURSION.

Narrative

I WAS PF; ENGAGED AUTOPLT AT 500 FT AGL. ON LOUPE 9 RBL TRANSITION. AS I STARTED TURN AT 1.8 DME; BAY APCH ADVISED US OF VFR TFC AT 3-4 O'CLOCK; LEVELING AT 4000 FT. WE HAD THE TFC ON TCASII BUT NO VISUAL BECAUSE WE WERE CLBING OUT OF OVCST. THE CAPT WAS TALKING TO BAY APCH ABOUT THE TFC AND CONCERNED THAT VFR TFC WOULD BE AT 4000 FT. BAY INFORMED HIM THAT THE ALT OF THE TFC WAS ASSIGNED BY APCH. PASSING THROUGH ABOUT A 030 DEG HDG AND 4000 FT; I SPOTTED THE TFC AND THE CAPT CALLED BAY APCH WITH TFC IN SIGHT. JUST AS THE RADIO XMISSION WAS ENDING THE AUTOPLT AND PMS WAS LEVELING THE ACFT AT 5000 FT AND 200 KTS. WITHOUT PAUSE BAY APCH CAME BACK WITH CLB AND MAINTAIN 12000 FT TO US. THE CAPT READ BACK 'ACR XYZ TO 12000 FT.' WE SET THE ALT ALERT TO 12000 FT AND CALLED OUT 12000 FT; SET AND ARMED. WE BOTH EXPECTED THIS SINCE WE WERE CLR OF THE TFC AND MANY TIMES THEY (BAY APCH) GIVE US AN EARLY CLB. PASSING THROUGH 6200 FT MSL AND ACCELERATING TO 250 KTS; BAY APCH CALLS US AND ASKS OUR ALT. WE RESPOND WITH 6300 FT FOR 12000 FT. BAY THEN SAYS YOUR ASSIGNED ALT WAS 5000 FT; AND TELLS US TO LEVEL OFF. WE LEVELED AT 7000 FT. ACR XAB WAS ALSO ON THE FREQ AHEAD OF US AND BAY APCH SAID THE CLB WAS FOR HIM. BUT THE CAPT AND I ARE POSITIVE BAY APCH SAID 'ACR XYZ.' WE FOLLOWED ALL PROCS OF OUR COMPANY'S ALT AWARENESS OF BOTH PLTS VERIFYING ALT SETTINGS. WE BELIEVE THAT ATC USED THE WRONG CALL SIGN. ATC HAS CONFUSED OUR CALL SIGNS A NUMBER OF TIMES IN THE PAST. MANY TIMES WE WILL HAVE 2 OR 3 ACR FLTS ON THE SAME FREQ WITH SIMILAR NUMBERS LIKE XYZ AND XAB OR ABC AND ACB. USUALLY ONE OF US WILL CATCH IT AND ALERT ATC OR EACH OTHER BEFORE SOMETHING HAPPENS. BUT THIS OCCURRENCE WAS SO CLR. THE CAPT WAS TALKING TO ATC AND WITHOUT PAUSE BAY APCH MADE AN ALT ASSIGNMENT; WHICH WE READ BACK AND HEARD NO OTHER XMISSION UNTIL BAY APCH QUERIED OUR ALT. COMING BACK FROM PDX WE GOT BAY APCH'S PHONE NUMBER AND CALLED TO TALK TO A SUPVR. SHE INFORMED US THAT NO RPTS WERE MADE. WE HAVE TOLD OUR COMPANY ABOUT THE CALL SIGN PROB BUT I HAVEN'T SEEN ANY CHANGES.