ATC ADVISED A B727 FLC OF TFC WHICH THEY WERE UNABLE TO ACQUIRE VISUALLY. WHEN THE TCASII RA COMMANDED A DSCNT; THE FLC FOLLOWED THE COMMANDS. THE OTHER ACFT WAS 200 FT LOWER THAN WHAT ATC ADVISED ACCORDING TO THE FLC'S TCASII.

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WE WERE INBOUND TO PHX ON THE SUNNS 1 ARR; LEVEL 9000 FT MSL. APCH CTL ADVISED THAT WE HAD VFR TFC AT 12 O'CLOCK. WE WERE UNABLE TO LOCATE VISUALLY; BUT DID LOCATE TFC ON TCASII. TCASII INITIALLY INDICATED TFC WAS AT 9300 FT; ALTHOUGH APCH SAID HE WAS AT 9500 FT. WE THEN RECEIVED A TCASII RA; WHICH COMMANDED A DSCNT OF UP TO 500 FPM (IVSI). WE ADVISED APCH OF THE TCASII RA AND COMMENCED A SHALLOW DSCNT. APCH RESPONDED BY ASKING THE VFR TFC TO VERIFY ALT. OUR TCASII INDICATED HE IMMEDIATELY CLBED BACK TO 9500 FT. WE NEVER LOCATED TFC VISUALLY; BUT TCASII SHOWED TFC PASSED DIRECTLY OVERHEAD. AFTER DSNDING TO 8800 FT; TCASII RA ENDED. WE CLBED BACK TO 9000 FT AND CONTINUED SUNNS 1 ARR. SUPPLEMENTAL INFO FROM ACN 331210: TCASII INITIALLY INDICATED TFC WAS AT 9300 FT ALTHOUGH APCH SAID HE WAS AT 9500 FT. WE THEN RECEIVED A TCASII ALERT; WHICH COMMANDED A DSCNT UP TO 500 FPM. WE ADVISED APCH OF THE TCASII ALERT. APCH RESPONDED BY ASKING THE VFR TFC TO VERIFY ALT. OUT TCASII INDICATED HE IMMEDIATELY CLBED BACK TO 9500 FT. THIS GOES TO SHOW HOW IMPORTANT TCASII IS ON ACFT; EVEN ON CARGO ACFT; WHICH ARE NOT REQUIRED UNDER FAR 121 LARGE ACFT TO HAVE TCASII. IT IS ALSO VERY IMPORTANT TO VFR PLTS WHEN FLYING THROUGH CLASS B AIRSPACE OR ANY OTHER ACTIVE AIRSPACE TO BE AWARE OF ALTIMETER SETTINGS.