Narrative:

Air carrier flight originated in ZZZ and terminated in atl. The captain was the PF. ZTL gave us a descent to 14000 ft MSL and switched us to atl approach control. We checked in with approach, advising them we were descending to 14000 ft with the current ATIS information. On the reply by approach, a descent clearance to 10000 ft was given but the call sign was not clear. To ensure it was our clearance, I asked the controller to confirm he wanted air carrier xyz to descend to 10000 ft. He replied affirmative. While leveling at 10000 ft, the approach controller queried our altitude. We replied we were leveling at 10000 ft. He immediately told us we had conflicting, co-altitude traffic at 12 O'clock position and directed us to climb immediately to 11000 ft. He stated we should have been at 12000 ft. He continued pointing out the traffic and telling us to climb. We complied. Passing through 10500 ft, the captain idented the traffic. Neither the captain nor I heard any TCASII warning. Once clear of the conflicting traffic, we were switched to another approach control frequency and continued to atlanta. The apparent cause of this potential conflict was the issuance and confirmation of a descent to 10000 ft by ATC for air carrier xyz. In the future, when a crew asks for a clearance confirmation, extra care is warranted to ensure a clearance is correctly communicated and received.

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Original NASA ASRS Text

Title: A B737-300 CREW WAS GIVEN CLRNC TO DSND TO AN OCCUPIED ALT IN A80 CLASS B AIRSPACE.

Narrative: ACR FLT ORIGINATED IN ZZZ AND TERMINATED IN ATL. THE CAPT WAS THE PF. ZTL GAVE US A DSCNT TO 14000 FT MSL AND SWITCHED US TO ATL APCH CTL. WE CHKED IN WITH APCH, ADVISING THEM WE WERE DSNDING TO 14000 FT WITH THE CURRENT ATIS INFO. ON THE REPLY BY APCH, A DSCNT CLRNC TO 10000 FT WAS GIVEN BUT THE CALL SIGN WAS NOT CLR. TO ENSURE IT WAS OUR CLRNC, I ASKED THE CTLR TO CONFIRM HE WANTED ACR XYZ TO DSND TO 10000 FT. HE REPLIED AFFIRMATIVE. WHILE LEVELING AT 10000 FT, THE APCH CTLR QUERIED OUR ALT. WE REPLIED WE WERE LEVELING AT 10000 FT. HE IMMEDIATELY TOLD US WE HAD CONFLICTING, CO-ALT TFC AT 12 O'CLOCK POS AND DIRECTED US TO CLB IMMEDIATELY TO 11000 FT. HE STATED WE SHOULD HAVE BEEN AT 12000 FT. HE CONTINUED POINTING OUT THE TFC AND TELLING US TO CLB. WE COMPLIED. PASSING THROUGH 10500 FT, THE CAPT IDENTED THE TFC. NEITHER THE CAPT NOR I HEARD ANY TCASII WARNING. ONCE CLR OF THE CONFLICTING TFC, WE WERE SWITCHED TO ANOTHER APCH CTL FREQ AND CONTINUED TO ATLANTA. THE APPARENT CAUSE OF THIS POTENTIAL CONFLICT WAS THE ISSUANCE AND CONFIRMATION OF A DSCNT TO 10000 FT BY ATC FOR ACR XYZ. IN THE FUTURE, WHEN A CREW ASKS FOR A CLRNC CONFIRMATION, EXTRA CARE IS WARRANTED TO ENSURE A CLRNC IS CORRECTLY COMMUNICATED AND RECEIVED.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.