ACR DSNDS FROM ASSIGNED; HAS TCASII RA.

Narrative

WE WERE INBOUND TO ATL ON THE ROME ARR ABOUT 20 NM NW ON THE 313 DEG RADIAL AT 12000 MSL 300 KIAS. I RECEIVED A RADIO CALL FROM APCH CTL WHICH WAS NOT FULLY UNDERSTOOD BY ME OR THE CAPT I ASKED APCH TO REPEAT THE XMISSION SINCE NEITHER ONE OF US WAS SURE WHAT WAS SAID. ATL REPEATED THE XMISSION BUT USED DIFFERENT TERMINOLOGY THIS TIME AND I UNDERSTOOD THAT WE WERE SUPPOSED TO SLOW TO 210 KIAS. I ALSO THINK THAT THE FIRST XMISSION WAS SOMETHING TO THE EFFECT OF 'SLOW TO 210 KIAS AT THE 10 DME FIX ON THE ROME ARR.' AS I WAS ROGERING THE XMISSION I NOTICED THE ACFT START A DSCNT. THE CAPT WAS FLYING AND HAD COMMANDED THE AUTOPLT TO START A DSCNT TO 10000 FT. I BELIEVE THE CAPT MISUNDERSTOOD THE XMISSION AND THOUGHT WE WERE TO DSND TO 10000 FT BY 21 NM. SINCE WE WERE PAST THIS POINT HE WAS DSNDING RAPIDLY. I QUICKLY SAID THAT I THOUGHT WE WERE NOT TO DSND AND HE SAID HE THOUGHT WE WERE. I QUICKLY CALLED APCH AND ASKED WHAT ALT HE WANTED US AT. HE RESPONDED 12000 FT AND THEN SAW THAT WE WERE AT 11500 FT AND TOLD US TO GO TO 11000 FT. WE WERE LEVELING AT 11000 FT AND ALMOST IMMEDIATELY WE GOT A TA WHICH QUICKLY WENT TO AN RA WITH A 'CLB; CLB' CALL ON THE TCASII. I OBSERVED AN MLG CLBING OUT OF ATL AT OUR 12 O'CLOCK AND APCHING HEAD-ON. THE CAPT FOLLOWED THE RA CLB WHICH INDICATED A 4000 FPM RATE OF CLB. I TOLD APCH CTL THAT WE WERE CLBING FOR AN RA TCASII ALERT. APCH GAVE US A L-HAND TURN FORM 133 DEGS TO 090 DEGS. THE RA WENT AWAY AFTER WE CLBED ABOUT 600 FT (AND THE MLG LEVELED OFF AT 10000 FT). WE NEVER GOT CLOSER THAN 1000 FT VERTICALLY AND 1/2 NM HORIZLY. AT THE TIME OF THE RA WE WERE DEFINITELY ON A COLLISION COURSE HAD WE NOT CLBED OR THE MLG NOT LEVELED OFF AT 10000 FT. THE CAUSE OF THIS INCIDENT WAS MISUNDERSTANDING ATC'S INSTRUCTIONS. CONTRIBUTING TO THIS MISUNDERSTANDING WAS HIGH COCKPIT NOISE LEVEL (300 KIAS AT 12000 FT); CONSTANT RADIO COMS ON A VERY BUSY ATL APCH CTL FREQ; LACK OF VERIFYING ALT CHANGE WITH THE OTHER CREW MEMBER (I NEVER READ BACK OR CONFIRMED THAT WE WERE CLRED TO 10000 FT). TCASII WORKED PROPERLY. HAD WE BEEN AT 12000 FT WE NEVER WOULD HAVE GOTTEN THE RA.