Date: 201408
Local time of day: 1801-2400
Location: MIA.Airport, FL
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E MIA
ATC / advisory: TRACON MIA
Second aircraft: Any Unknown or Unlisted Aircraft Manufacturer
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Human-Machine Interface; Training / Qualification; Communication Breakdown
Second reporter function: Captain; Pilot Not Flying
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft TA; Automation Aircraft RA
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: While on the HILEY2 arrival into MIA the Captain (pilot not flying) requested the ILS to either 8R or 9L for operational needs (as the airplane needed an autoland). A few moments later while level at 14;000 feet; we were assigned the ILS 8R and vectored off the arrival to a heading of 220 and given a descent. With the autopilot flying; I selected 220 and heading select; then dialed in the altitude I thought I heard (12;000) in the altitude window and selected FLCH. The Captain questioned the altitude with me because he felt he had heard the controller give us 10;000 feet. We both agreed we should probably verify this discrepancy but the frequency was too overloaded to get a word in edgewise. Additionally; the Captain was confident that the altitude was in fact 10;000 feet because he had read the altitude back to the controller and the controller did not bother to correct him. So between these two things working against my better judgment; I then selected 10;000 in the altitude window.As we descended through approximately 11;000 feet; we received [a TA] for traffic. I could tell from the screen the traffic was climbing but was unable to visually identify the traffic in the night sky. About the time we reached approximately 10;600 feet we received [an RA] at which point I turned off the automation and hand flew the airplane; performing the escape maneuver per the RA guidance. Within a few seconds the RA was averted and; while I was never actually able to visually identify the other aircraft; it didn't appear that any imminent danger had occurred. The Captain immediately informed the controller of our TCAS avoidance situation and the controller advised there was no problem as the aircraft was below us at 10;000 feet. This; of course; confused us a bit since our aircraft had descended to as low as 10;300 feet during the RA maneuver; and now we were unsure if we were still expected to descend to 10;000 feet or not. Then the controller came back to us and instructed us to climb to 11;000 feet which we did. I feel this is a classic case of a busy controller/radio frequency combined with some confusion in the cockpit about ATC instructions which; in hindsight; should have been verified with the controller prior to descending below 12;000' despite the radio congestion. Frankly as I think back on this incident; I'm not sure if the controller had initially given us 12;000 feet; 11;000 feet; or 10;000 feet. Either the 12;000 feet or 11;000 feet altitudes would have been the 'more correct' altitudes to keep us clear of this traffic; and if in fact we were given the 10;000 feet altitude; then I feel this was given to us in error. Regardless of any errors on part of the controller; we as a crew could have done a better job to verify. The good news is that the system worked as it should; and TCAS did its job; along with proper training in RA maneuvers.


Second narrative: [The report narrative contained no additional information.]


Synopsis: When the flight crew of a B757-200 disagreed as to their cleared altitude and was unable to get through a busy TRACON frequency to verify it they opted to take the PNF's recollection and continued their descent to 10;000 rather than the 12;000 understood by the First Officer.  A TCAS RA occurred and was complied with in the vicinity of 10;600.