CAPT OF AN MD88 UNDERSHOT ASSIGNED ALT DURING DEP DUE TO A MISUNDERSTANDING OF THE ALT AND SUBSEQUENT ATTEMPTS TO CLARIFY THE CORRECT ALT WITH ATC DUE TO FREQ CONGESTION.

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DURING THE DEP/CLBOUT; WE WERE FOLLOWING ANOTHER MD88 4-5 MI IN TRAIL. WE WERE LEVEL AT 12000 FT; HDG APPROX 110 DEGS. THE LEAD ACFT WAS CLBING ABOVE US ON A 130-140 DEG HDG; DRIFTING TO THE S OF OUR COURSE. THE DEP CTLR WAS VERY BUSY WHEN HE ISSUED THE LEAD ACFT A TURN TO THE NE THAT THEY MISSED AND CONTINUED ON THEIR 130-140 DEG HDG. MOMENTS LATER; THEY RECEIVED AND ACKNOWLEDGED ANOTHER CLRNC TO TURN TO THE NE. BECAUSE OF THEIR NEW HDG AND EARLIER MISSED CALL; LATERAL SEPARATION BEGAN TO NARROW BTWN THE 2 ACFT (VERT SEPARATION OK BUT I DON'T KNOW BY HOW MUCH). WE WERE THEN ISSUED A TURN AND CLB BUT THE CALL SIGN WAS PARTIALLY BLOCKED AND I READ BACK 'CALL SIGN BLOCKED; UNDERSTAND CLRED HDG XXX DEGS AND 14000 FT.' THE CTLR ANSWERED AFFIRMATIVE. THE CAPT SAID HE UNDERSTOOD 13000 FT AND LEVELED AT 13000 FT. I THOUGHT HE MIGHT JUST BE RIGHT SINCE NOW WE WERE PRETTY CLOSE TO THE LEAD M88. I COULD NOT CLARIFY THIS WITH DEP BECAUSE OF FREQ CONGESTION. WE WERE THEN GIVEN A FREQ CHANGE TO ZTL. I DID NOT CLR UP THE CONFUSED ALT THEN BECAUSE THE DEP CTLR WAS VERY BUSY; THERE DID NOT APPEAR TO BE AN IMMEDIATE THREAT TO OUR ACFT AND I THOUGHT ZTL COULD CLR US TO 14000 FT AND ABOVE IF IN FACT THE CAPT WAS RIGHT. THE CTR CTLR WANTED TO KNOW; UPON CHK-IN; IF WE HAD BEEN CLRED TO 14000 FT. SINCE WE WERE UNSURE HE HAD TO CALL ON THE LAND LINE TO GET US TO 14000 FT. WE DRONED ON FOR AT LEAST A MIN IN THIS VERY UNCOMFORTABLE CONFUSED STATE. THE NEXT TIME I WILL CLR THIS UP WITH THE CTLR WHO ISSUED THE CLRNC; RATHER THAN RELYING ON THE NEXT GUY/GAL TO FIX SOMETHING THEY MAY NOT HAVE THE INFO OR LATITUDE TO ACT ON.