B737 ARR TO PHX EXPERIENCED TCASII RA WHEN ZAB ISSUED ALT CLRNC IN CONFLICT WITH PRECEDING B757 AT FL240.

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ON THE ARLIN ONE ARR TO PHX. ATC REQUESTED BEST FORWARD SPD. THEY WERE TRYING TO GET US BY ANOTHER CARRIER B757 THAT WAS SPD RESTR AHEAD OF US; BUT SAME DIRECTION. WE WERE AT FL250. ATC ISSUED A CLRNC FOR US TO DSND TO FL240. THE CAPT BEGAN A SLOW DSCNT. I NOTICED THAT THE B757 TFC HAD NOT YET PASSED UNDER US AND BEGAN TO WONDER WHAT ALT HE WAS AT. LOOKING AT THE TCASII; I REALIZED THAT HE WAS AT FL240 ONLY 1000 FT BELOW US. AS I SPOTTED THE TFC AND WAS ABOUT TO BEGIN A STATEMENT TO THE CAPT ABOUT THE TFC; THE TCASII RESPONDED WITH AN RA. THE CAPT IMMEDIATELY RESPONDED WITH A CLB BACK TO FL250. WE HAD DSNDED NO LOWER THAN 24700 FT. WE ASKED ATC ABOUT OUR CLRNC. HE RESPONDED THAT HE NEEDED US AT FL250. WE STAYED AT FL250 TILL CLR OF TFC AND THEY REISSUED THE DSCNT CLRNC. WE WERE LATER GIVEN A NUMBER TO CALL ATC. ON THE GND IN PHX; THE CAPT CALLED ATC. WE WERE INFORMED THAT THERE WAS SOME CONFUSION ON THE PART OF ATC AND THAT THEY WOULD REVIEW WHAT HAD OCCURRED AND THAT WE NEEDED TO CALL BACK. UPON THE CAPT'S CALLBACK; HE WAS TOLD THAT THE CTLR THOUGHT HE HAD TOLD US TO REMAIN AT FL250 UNTIL CLR OF THE TFC. THEY WERE NOT ABLE TO CONFIRM THIS LISTENING TO THE TAPE; ONLY THAT HE HAD CLRED US FROM FL250 TO FL240. HE ALSO INFORMED THE CAPT THAT BETTER SITUATIONAL AWARENESS ON OUR PART COULD HAVE AVOIDED THIS SIT. I FELT THAT OUR SITUATIONAL AWARENESS KEPT IT FROM BEING WORSE THAN IT COULD HAVE BEEN. THEY INDICATED CLRLY; THAT IT WAS THE FAULT OF THE CTLR AND THAT NO FURTHER ACTION WAS REQUIRED FROM US. I FEEL THAT CLOSER ATTN TO THE ALT OF OUR TFC COULD HAVE PREVENTED US FROM BEGINNING THE DSCNT AT ALL. HOWEVER; I ALSO FEEL THAT WE DID PICK UP THE ERROR IN A TIMELY MANNER AND RESPONDED WITH APPROPRIATE ACTION.