LTT INSTRUCTED TO MAKE MISSED APCH DUE TO PROX OF ANOTHER ACFT DURING RWY CHANGE. APCH USED WRONG CALL SIGN DURING ENSUING INSTRUCTIONS.

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FLT WAS ENRTE FROM PHL TO EWR. EWR WAS LNDG AND DEPARTING ON RWYS 22R&L. THE WINDS AT EWR WERE 350 DEGS AT 10 KTS. WE WERE TOLD TO EXPECT THE ILS RWY 22L BUT THEN TWR DECIDED TO SWITCH AROUND TO THE 4'S; SO APCH TOLD US TO EXPECT THE ILS RWY 4R. APCH CTL (128.55) GAVE US VECTORS FOR THE APCH AND EVERYTHING WAS GOING NORMAL. THE CAPT AND I SET UP FOR THE APCH AND WENT THROUGH THE CHKLIST. APCH GAVE US AN INTERCEPT HDG AND THE LOC WAS COMING IN AS EXPECTED. APCH CLRED US FOR THE ILS RWY 4R AND SWITCHED US OVER TO TWR FREQ ABOUT 9 MI S OF THE FIELD. I CHKED IN WITH TWR AND HE SEEMED VERY CONFUSED ON WHO WE WERE AND ASKED US TO IDENT FOR HIM. WE COMPLIED WITH HIS REQUEST AND HE THEN IMMEDIATELY AND FRANTICALLY GAVE US A 090 DEG HDG OFF THE LOC AND TOLD US TO DSND TO 1500 FT (FROM 2000 FT). HE ALSO TOLD US HE HAD A DC-10 AT 2000 FT AND 2 MI. THE CAPT PUT THE ACFT INTO THE TURN WHILE PULLING OFF THE PWR TO START HIS DSCNT. (THIS MANEUVER SET OFF THE GPWS ALT AND BANK ANGLE WARNINGS.) ABOUT 15- 20 SECONDS LATER; THE TWR TOLD US THAT THE DC-10 (LATER WE FOUND OUT IT WAS AN AIRBUS) HAD PASSED BEHIND US AND NOW WAS AT 2500 FT. TWR GAVE US A VECTOR AND TOLD US TO CLB TO 3000 FT THEN TOLD US TO EXECUTE THE PUBLISHED MISSED APCH PROC. WHEN WE CHKED IN WITH APCH CTL AGAIN (119.2) HE; TOO; SEEMED A LITTLE CONFUSED ON WHO WE WERE (ACR AX). HE ASKED US TO IDENT FOR HIM AND THEN HE STARTED CALLING US ACR AB; AND TOLD US TO CLB TO 5000 FT ON A 030 DEG HDG. THE CAPT AND I WERE SKEPTICAL SO I ASKED TO CLARIFY ACR AX SQUAWK CODE AS OUR OWN. HE CAME BACK SAYING 'I DON'T CARE WHAT YOUR SQUAWK IS!' AND THEN REPEATED ACR AB CLB AND MAINTAIN 5000 FT ON A 030 DEG HDG. QUESTION: WHO ARE WE? ACR AX AS FILED OR ACR AB? WE TOOK THE CALL AND STARTED UP TO 5000 FT AND TURNED TO THE HDG. WE WERE VECTORED AROUND FOR ANOTHER ILS; THE WHOLE TIME BEING CALLED ACR AB. WE LANDED WITHOUT FURTHER INCIDENT. I BELIEVE A FAILURE IN COM BTWN APCH CTL AND TWR OCCURRED ON ARR AND ALSO ON MISSED APCH. WHY WERE WE CLRED THE ILS RWY 4R WHEN TWR WAS STILL SENDING OFF ACFT N RWYS 22R&L? AND WHY WAS OUR FLT NUMBER CHANGED ON MISSED APCH? THE ONLY ANSWER I HAVE IS A LACK OF COM BTWN TWR AND APCH CTL. SUPPLEMENTAL INFO FROM ACN 278042: AFTER CLBING TO 5000 FT I ASKED FOR A PHONE NUMBER TO CALL WHEN WE GOT ON THE GND! AFTER TALKING TO NEW YORK APCH ON THE PHONE AND TWR (EWR) SUPVR ON THE PHONE; I CALLED OUR DIRECTOR OF OPS TO MAKE THEM AWARE OF THE NEAR MIDAIR. THE NEXT DAY I CALLED EWR TWR AND FILED A NEAR MISS RPT. AS OF NOW; WE ARE WAITING TO HEAR THE INVESTIGATING RPTS.