REPLY TO WRONG CALL SIGN SPONSORS AN UNAUTH TKOF.

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WE WERE CLRED FOR POS AND HOLD ON RWY 17 AT PHL. WHILE IN POS; A TKOF CLRNC WAS ISSUED TO ANOTHER OF OUR COMPANY ACFT ON RWY 27L. THIS AIRPLANE HAD SEVERAL OF THE SAME DIGITS AS OUR FLT NUMBER. THE FO ON MY ACFT ACKNOWLEDGED THE TKOF CLRNC WITH OUR FLT NUMBER AND COMMENCED THE TKOF ROLL. AS WE WERE LIFTING OFF; I OBSERVED AN ACFT XING OUR DEP PATH IN THE PROCESS OF LNDG ON RWY 27R. I ALSO OBSERVED AN ACFT DEPARTING RWY 27L WHICH TURNED OUT TO BE OUR COMPANY ACFT WITH THE SIMILAR CALL SIGN WHO HAD ACTUALLY BEEN CLRED FOR TKOF. I CONTACTED TWR AND INFORMED HIM WE WERE AIRBORNE. APPARENTLY; THE FO HAD ACKNOWLEDGED THE TKOF CLRNC SIMULTANEOUSLY WITH THE OTHER COMPANY ACFT. HIS XMISSION BLOCKED OURS. SEVERAL FACTORS TO BE CONSIDERED: 4 DIGIT FLT NUMBERS ARE A CONSTANT SOURCE OF CONFUSION. OCCASIONALLY YOU WILL FIND 2 DIFFERENT CARRIERS ON FREQ WITH THE SAME CALL SIGN. AT THE HUB ARPTS ATC SOMETIMES IS HANDLING MULTITUDES OF SAME COMPANY FLTS; SOMETIMES WITH SEVERAL IDENTICAL DIGITS. ATC AND PLTS NEED TO BE REMINDED TO BE MORE VIGILANT. SIT OCCURRED AT NIGHT; LAST LEG OF A LONG DAY. COMPANY DOWNSIZING; FO PRE-OCCUPIED WITH IMPENDING FURLOUGH. AS PNF; I WAS SUPPOSEDLY WORKING RADIOS. WHEN TKOF CLRNC WAS GIVEN; I HESITATED IN THE ACKNOWLEDGEMENT BECAUSE IT DIDN'T SOUND RIGHT. THE FO; BECAUSE WE WERE IN A KNOWN DEAD SPOT FOR COM ON ARPT; ASSUMED I HAD MISSED THE CLRNC; AND RESPONDED TO IT. BECAUSE THERE WAS NO RESPONSE FROM TWR I ASSUMED I HAD HEARD IT WRONG. SUPPLEMENTAL INFO FROM ACN 279271: WE SAT ON THE RWY FOR APPROX 2 MINS; WHEN I HEARD TWR SAY OUR COMPANY NAME; OUR 4 DIGIT FLT NUMBER; AND 'CLRED FOR TKOF.' IT WAS MY LEG; AND IT'S COMPANY POLICY FOR THE PNF TO WORK THE RADIO ONCE WE ARE ON THE RWY. HOWEVER THE CAPT FAILED TO ANSWER THE CALL. THINKING THAT HE MAY NOT HAVE BEEN ON BOTH COMS AND MISSED THE CALL; I READ BACK THE CLRNC; THEN LOOKED TO THE CAPT WHO NODDED; AS I ADVANCED THE PWR LEVERS. AFTER WE CLEANED UP THE AIRPLANE; WE CONTACTED THE TWR WHO TOLD US THAT WE HAD ONLY BEEN CLRED FOR POS AND HOLD. HE CONFIRMED THE TKOF CLRNC WAS FOR ANOTHER COMPANY ACFT; ON RWY 27L. THE FIRST AND LAST DIGITS OF THE OTHER ACFT'S CALL SIGN WERE THE SAME AS OUR OWN. REACHING CRUISE I ASKED THE CAPT IF HE WAS ON BOTH COM RADIOS; AND IF HE ALSO UNDERSTOOD THE CLRNC TO BE FOR US. HE TOLD ME THAT HE WAS ON BOTH RADIOS; BUT HAD SERIOUS DOUBTS THAT THE CLRNC WAS OURS! HE WITHHELD COMMENT HOWEVER; WHEN TWR DID NOT CHALLENGE MY READBACK. AN ADDED FACTOR WAS OVER THE DURATION OF THE 2-DAY TRIP; THE CAPT MISSED A FEW CALLS. I SUSPECTED THAT THIS WAS THE CASE AGAIN. THE MOST GRIEVOUS ERROR WAS THE CAPT'S FAILURE TO STOP THE TKOF FROM HAPPENING IF HE HAD ANY MISGIVINGS; LET ALONE A 'SERIOUS DOUBT' ABOUT THE VALIDITY OF OUR TKOF CLRNC. I WAS 100 PERCENT SURE IN MY MIND; THAT THE TKOF CLRNC ISSUED WAS FOR US. HOWEVER; IF THE CAPT HAD SAID EVEN; 'I'M NOT SO SURE...' I WOULD HAVE LOCKED UP THE BRAKES.