B757 FLC WITH DSCNT CLRNC MISUNDERSTOOD ALT ASSIGNMENT AND READ BACK WHAT THEY HEARD WHICH WAS NOT CAUGHT BY THE CTLR UNTIL CTLR SAW THEM DSNDING THROUGH THE ASSIGNED.

Narrative

WE WERE ON VECTORS FOR THE ILS RWY 1R AT IAD; 4000 FT MSL ASSIGNED. WE WERE THEN CLRED TO INTERCEPT THE LOC FOR RWY 1R. SHORTLY THEREAFTER WE WERE CLRED (OR THOUGHT WE WERE CLRED) TO 'CROSS MOSBY AT 2000 FT; CLRED ILS RWY 1R APCH.' WE THEN COMMENCED OUR DSCNT TO 2000 FT. AT 2300 FT APCH CTL REQUESTED OUR ALT. WE RESPONDED '2300 FT.' APCH RESPONDED THAT WE SHOULD CLB TO 3000 FT AND THAT OUR CLRNC WAS TO CROSS MOSBY AT 3000 FT. WE STARTED OUR CLB BACK TO 3000 FT. AT 2500 FT; APCH CTL REQUESTED OUR CURRENT ALT; TO WHICH WE RESPONDED 2500 FT. APCH RESPONDED THAT WE COULD MAINTAIN OUR CURRENT ALT AND WERE THEN CLRED AGAIN FOR THE ILS RWY 1R APCH; WHICH WAS COMPLETED WITHOUT FURTHER INCIDENT. THE CAPT AND I BOTH BELIEVE THAT THE CLRNC WAS TO CROSS MOSBY AT 2000 FT. I KNOW I RESPONDED WITH 2000 FT AND SET IT IN THE ALT WINDOW; WHICH THE CAPT CONFIRMED. IT IS FRUSTRATING THAT IF OUR RESPONSE TO THE CLRNC WAS INCORRECT; THAT APCH CTL (WHICH WAS NOT VERY BUSY AT THE TIME) DID NOT CATCH AN INCORRECT READBACK OF A CLRNC.