ALTDEV WHILE 1 MEMBER OF A 3 MEMBER FLC IS FLYING AND WORKING THE RADIOS.

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DURING CLBOUT; DAY-MCI; BTWN 14000 FT AND 16000 FT; THE CAPT TOLD ME TO MONITOR ZID AS HE NEEDED TO RESOLVE A QUESTION WITH OUR FLT PLAN WITH DISPATCH DAYTON. I THEN HAD CTL OF THE AIRPLANE AS WELL AS THE RADIO. THE CTLR BEGAN TO STEP CLB US AS RESULT OF TFC ABOVE US; IN FACT THE CTLR ADVISED US THAT WE MIGHT SEE TFC AT OUR 10 O'CLOCK POS. I SPOTTED TFC WHILE LEVEL FL190; BOTH AIRPLANES WERE THEN CLR OF CLOUDS. I WAS THEN GIVEN WHAT I UNDERSTOOD TO BE A CLB CLRNC FROM FL190 TO FL220; I BELIEVE THAT'S WHAT I READ BACK BECAUSE I DELIBERATELY SET FL220 IN ALT ALERTER AND COMMENCED CLBING THE AIRPLANE. APPROX FL200 THE REST OF THE CREW JOINED ME. THE CAPT REASSUMED CTL OF THE RADIO COM. WE ALL HAD VISUAL CONTACT WITH OUR 10 O'CLOCK TFC WHO WAS STILL ABOVE US. I DON'T BELIEVE WE WERE GIVEN THE ALT OF THE TFC; HE WAS ALSO APPARENTLY CLBING. APPROX FL210 THE CTLR QUERIED US AND THEN ISSUED INSTRUCTIONS FOR AN IMMEDIATE DSCNT BACK TO FL200. THE CTLR SUBSEQUENTLY INFORMED US OF A POSSIBLE 'DEV.' SUPPLEMENTAL INFO FROM ACN 298218: MY SUBSEQUENT REVIEW OF THE ATC TAPE REVEALED THAT HE READ BACK FL200 AND THEN SET FL220 IN THE ALT ALERTER. I WOULD ADVISE FLCS TO DO THE FOLLOWING: 1) AVOID; WHEN POSSIBLE; NON-ATC COMS DURING PERIODS OF HIGH ATC ACTIVITY (SUCH AS WHEN RECEIVING STEP-CLB ALT ASSIGNMENTS). 2) USE THE AUTOPLT TO REDUCE COCKPIT WORKLOAD WHEN 1 PLT IS OFF THE ATC FREQ. 3) USE A 3RD CREWMEMBER (IF AVAILABLE) TO ASSUME ALT CONFIRMATION DUTIES WHEN 1 PLT IS OFF THE ATC FREQ. SUPPLEMENTAL INFO FROM ACN 298673: FO THOUGHT CLRNC WAS TO FL220 NOT FL200. THIS ERROR MAY HAVE BEEN PREVENTED IF I; FE; HAD SWITCHED TO ATC FREQ AND BACKED UP FO WHILE CAPT WAS ON COMPANY FREQ.