ACFT USING WRONG CALL SIGN.

Narrative

OUR FIRST FLT OF THE DAY WAS FLT XAB; ATL TO CLT. IT PROCEEDED NORMALLY AND WE CHANGED PLANES IN CHARLOTTE. OUR NEXT FLT WAS FLT XA; CLT BACK TO ATL. WE GOT THE CLRNC; FLT PLAN; TAXI OUT AND TKOF NORMALLY AS FLT XA AND PROCEEDED TO OUR DEST OF ATL. WHEN WE WERE ON DEP CTL; THE FO STARTED TO USE THE CALL SIGN XAB OF OUR PREVIOUS FLT BY ACCIDENT INSTEAD OF FLT XA. (BY THE WAY; FLT XAB WAS TO CONTINUE FROM CHARLOTTE TO GSP.) AT THE SAME TIME; WHICH HAPPENED TO BE ALONG OUR SAME FILED ROUTING TO ATL. AS WE PROCEEDED ALONG OUR FILED ROUTING TO ATL; ATC LEVELED US AT 10000 FT AND TOLD US TO CONTACT GSP TWR. WE THEN REALIZED ATL THOUGHT WE WERE LNDG AT GSP AND WE TOLD THEM WE ACCIDENTALLY STARTED USING THE WRONG CALL SIGN. AS WE WERE NOT OFF COURSE OR CAUSING A CONFLICT; THEY UNDERSTOOD THE MISTAKE AND HAD US DO A 360 DEG TURN TO HELP SEQUENCE INTO THE TFC FLOW TO ATL IN THE FILED STAR. THEY CLBED US UP TO ALT AND THE REST OF THE FLT PROCEEDED NORMALLY. THE MISTAKE WAS CAUSED BY ACCIDENTLY LOOKING AT THE WRONG CALL SIGN ON THE YOKE CLIP BOARD AND WE OR ATC DIDN'T CATCH AT FIRST THAT WE STARTED USING THE WRONG CALL SIGN. MORE VIGILANCE IN OUR READBACKS AND LARGER NUMBERS WRITTEN IN FRONT OF US ON OUR CLIP BOARD MIGHT HAVE PREVENTED US FROM ACCIDENTALLY USING THE WRONG ONE PART WAY THROUGH THE FLT. THE OTHER FLT NUMBER WAS WRITTEN ON THE YOKE PAPER EARLIER AND WASN'T CROSSED OUT PROPERLY. SUPPLEMENTAL INFO FROM ACN 268971: I GOT OUR CLRNC VIA PDC FOR OUR FLT FROM CLT TO ATL AS FLT XA. SOMEWHERE BTWN PUSHBACK AND DEP I STARTED USING OUR OLD FLT NUMBER FROM THE PREVIOUS LEG; FLT XAB.