MLG WITH NEW FO FILES FLT PLAN USING WRONG FLT NUMBER. CORRECTS AFTER CONTACTING DEP.

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I ASSUMED A CALL SIGN WHICH WAS NOT APPROPRIATE FOR MY FLT NUMBER! I WAS THE CAPT OF AIRLINE FLT XYZ ELP TO LAS; TURN AROUND FOR OUTBOUND FLT XXA KAS-PHX. MY FO WAS ON HIS FIRST FLT OF HIS AIRLINE CAREER. WE HAD ALREADY HAD A STRESSFUL DAY WITH AN ACFT SWAP IN ELP. WE WERE 1 HR LATE. THE FO ASKED FOR A CLRNC FOR XYZ WHILE I WAS OFF THE AIRPLANE IN LAS TO HANDLE A PAX PROB. OF COURSE; NO FLT PLAN EXISTED FOR XYZ ONLY XXA. THE FO ASSUMED THAT THE CLRNC HAD 'DROPPED' BECAUSE WE WERE LATE. HE DID NOT RECOGNIZE THAT WE HAD CHANGED FLT NUMBER. WHEN I RETURNED; HE READ ME THE CLRNC LESS THE FLT NUMBER SINCE HE HAD REFILED. THE CLRNC MATCHED OUR COMPANY RTE OF FLT. WE STARTED ENGS AND TAXIED OUT AS FLT XYZ. I WAS MORE CONCERNED WITH WATCHING HIM ON HIS FIRST REVENUE FLT THAN I WAS LISTENING FOR THE APPROPRIATE CALL SIGN. WHEN LAS TWR CLRED US FOR TKOF; I RECOGNIZED THE DISPARITY. HOWEVER; ROLLING DOWN THE RWY IS NOT THE TIME TO CHALLENGE THE FO OR THE TWR FOR CLARIFICATION. WHEN TWR HANDED US OFF TO LAS DEP; I QUERIED TWR AS TO WHAT HE SHOWED FOR A CALL SIGN. HE SHOWED XYZ. I WENT OVER TO DEP; ASKED WHERE XYZ WAS FILED TO; AND TOLD HIM TO REFLECT A CHANGE OF CALL SIGN TO XXA. HE DID SO; AND WAS VERY GRACIOUS ABOUT IT. UPON ARR IN PHX; I CALLED THE LAS TWR ON THE PHONE AND DISCUSSED THE INCIDENT WITH THE TWR CHIEF. HE INDICATED IT HAD NOT CREATED A PROB. IN THE FUTURE I WILL ASK THE CTLR/FO TO READ CLRNC TO ME; INCLUDING THE FLT NUMBER!