CRJ700 DEPARTS WITH INCORRECT BEACON CODE CAUSING CONFUSION FOR CONTROLLER IN TRAINING.

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RECEIVED A BEACON CODE AFTER RECEIVING THE DEP CLRNC. WHILE ON TXWY M; AND WAITING IN LINE; MY FO RESPONDED TO GND CTL ON A SQUAWK CODE CHANGE. I HEARD THE CHANGE PROVIDED BY GND CTL BUT WAS NOT SURE IF IT WAS FOR US. I QUESTIONED MY FO IF THAT WAS FOR US AND HE CONVINCED ME IT WAS. WE CONTINUED AND WERE CLRED TO TAXI INTO POS AND HOLD AND LATER CLRED FOR TKOF. I DID NOT THINK OF IT ANY FURTHER WITH THE LAST 2 CALLS DUE TO OUR ORIGINAL CALL SIGN WAS BEING USED. WE DEPARTED OFF OF RWY 9L; RADIO TFC WAS HVY AND WE WERE NOT GIVEN OVER TO DEP IN THE NORMAL TIME AS USUAL. THINKING TWR WAS TOO BUSY; OR THEY THOUGHT WE WERE HANDED OFF; I SWITCHED FREQS TO DEP AND RECEIVED NO ANSWER. I SWITCHED BACK OVER TO TWR AND ASKED FOR THE FREQ. I WAS ABLE TO MAKE CONTACT WITH DEP AT THIS TIME. UNFORTUNATELY; HE WAS LOOKING FOR ACFT Y AND ASKED WHAT RWY WE DEPARTED. I EXPLAINED WE WERE OFF OF RWY 9L. HE SEEMED CONFUSED UNTIL ANOTHER CTLR CAME OVER THE RADIO AND ASKED FOR AN IDENT. THIS CTLR REALIZED WE WERE ON THE WRONG SQUAWK AND GAVE US THE CORRECT ONE. SHE THEN GAVE US A HDG TO INTERCEPT THE DEP. WE PROCEEDED WITHOUT INCIDENT AND I WAS ABLE TO CALL DEP CTL VIA TELEPHONE ONCE WE LANDED. I TALKED WITH A SECTOR SUPVR WHO WAS AWARE OF THE EVENT. WHEN I TOLD THIS PERSON; WHOSE NAME I DID NOT OBTAIN; ABOUT THE SITUATION SHE ADVISED THAT IT WAS NOT A BIG DEAL AND THE FIRST CTLR WAS IN TRAINING. THE SECOND CTLR WHO TOOK OVER WAS THE TRAINER. I WAS ADVISED THAT OUR FLT WAS WHERE IT WAS SUPPOSED TO BE BUT THE TRAINING CTLR DID NOT KNOW WHO WE WERE DUE TO THE INCORRECT CODE.