FLC DEV OCCURS WHEN FO STARTS ACFT DOWN FROM FL350 TO FL250. HE HAD JUST AFFIRMED A SPD REDUCTION TO 250 KTS AND THEN SAID; 'OUT OF FL350 FOR FL250.' FO CRITICIZES CTLR'S PHRASEOLOGY. CAPT WAS JUST RETURNING TO COCKPIT.

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INBOUND TO ATL ON J48. WE WERE 20 MI IN TRAIL OF OTHER COMPANY FLT. I (CAPT) LEFT COCKPIT TO USE THE LAVATORY. WHEN I RETURNED; THE FO HAD LEFT FL350 FOR FL250. CTLR HAD TOLD OTHER FLT TO MAINTAIN 250 KTS WHEN ABLE. HE THEN SAID; 'OUR CALL SIGN; SAME FOR YOU; MAINTAIN TWO-FIVE- ZERO.' THE FO HEARD THIS AS A CLRNC TO FL250 AND STARTED DSCNT. PASSING THROUGH FL340 ATC CALLED AND SAID; 'OUR CALL SIGN; VERIFY LEVEL FL350.' WE ANSWERED; 'NEGATIVE; OUT OF FL340 FOR FL250.' CTLR SAID; 'CLB TO FL350 AND TURN R TO 270 DEGS; TFC AT 11 O'CLOCK AND 10 MI.' WE COMPLIED AND WHEN LEVEL AT FL350; WE WERE VECTORED BACK TO J48 AND CONTINUED TO ATL. SUPPLEMENTAL INFO FROM ACN 379675: CTLR: 'OUR FLT THE SAME FOR YOU AS WELL; MAINTAIN 250.' FO: 'OUR FLT; ROGER MAINTAIN 250 KTS WHEN ABLE; OUT OF 350 FOR 250.' SUPPLEMENTAL INFO FROM ACN 379738: THE CAPT HAD LEFT THE COCKPIT FOR A BATHROOM BREAK AND WAS JUST RETURNING. I WAS OPENING THE DOOR FOR HIM AND MISSED THIS RADIO CALL. THE FO ANSWERED THE CALL AND UNDERSTOOD THAT WE WERE CLRED TO FL25O; AND BEGAN A DSCNT.