B190 FO DOES NOT CHALLENGE CAPT'S TKOF CLRNC READBACK TO SIMILAR SOUNDING CALL SIGN. SHORTENED CALL SIGNS ARE UTILIZED BY PHL TWR.

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I WAS THE FO AND THE PF. THE CAPT WAS THE PNF AND WAS HANDLING THE RADIOS. WE HAD BEEN ON DUTY FOR 12.5 HRS; HAVING FLOWN 7 PREVIOUS LEGS; ALL TO INST APCHS IN MODERATE (CEILING ABOUT 800 FT) IMC. I HAD FLOWN 3 OF THE PREVIOUS 7 LEGS. THIS WAS OUR LAST LEG. OUR FLT NUMBER WAS ACR X1. PHL CLRNC DELIVERY OF OUR COMPANY. I DO NOT KNOW WHICH; SHORTENED THE CALL TO 'ACR X1.' I UNDERSTAND THAT THIS IS COMMON FOR A FLT DEPARTING PHL. ARRIVING FLT NUMBERS ARE NOT SHORTENED. UNBEKNOWNST TO US; THERE WAS ANOTHER COMPANY ACFT TO DEPART AT THE SAME TIME (XD58) WHICH WAS FLT 2 AND DESIGNATED AS 'ACR X2.' AFTER A WAIT OF 5 MINS OR SO; AT THE RWY HOLD SHORT LINE; WE WERE TOLD TO TAXI INTO POS AND HOLD RWY 9L. THE CAPT COMPLIED; AND TURNED THE CTLS OVER TO ME; WITH MY ACKNOWLEDGEMENT. ALL CHKLISTS HAD BEEN COMPLETED. THERE WAS QUITE A LINE OF ACR TFC WAITING TO DEPART FROM RWY 9L AND WE WERE ABOUT FIFTH OR SIXTH CLRED ONTO THE RWY. WE HELD IN POS FOR MORE THAN 1 MIN. TO THE BEST OF MY RECOLLECTION; THIS IS WHAT HAPPENED NEXT. PHL TWR SAID 'ACR X2 CLRED FOR TKOF RWY 9L.' I HEARD X2 AND NOT X1. SINCE WE WERE ON RWY 9L; I ASSUMED IT WAS MEANT FOR US. UP TO THIS POINT IN TIME; WE HAD NOT HEARD ANY CALLS FOR ACR X2. THE CAPT ACKNOWLEDGED BY SAYING 'ACR X1 CLRED FOR TKOF RWY 9;' AND ASKED FOR AN INITIAL ALT. THE PHL TWR CTLR SAID (RATHER TERSELY) '5000 FT.' THE CAPT ACKNOWLEDGED THE ALT ASSIGNMENT BY SAYING 'ACR X1 5000 FT;' AND TOLD ME TO GO; WHICH I DID; DESPITE MY FEELING THAT SOMETHING WAS WRONG. AS WE BEGAN OUR TKOF ROLL; THE PHL TWR CTLR CAME ON THE RADIO AND WAS KIND OF SPITTING AND SPUTTERING; BUT GAVE NO DIRECTIVES TO ANYONE. THIS CAUGHT THE ATTN OF BOTH MYSELF AND THE CAPT. NONETHELESS WE CONTINUED. TKOF WAS NORMAL. AFTER LIFTOFF; AND XING THE INTERSECTING RWY; THE PHL TWR CTLR SAID 'ACR X1 TURN L HDG 050 DEGS; CONTACT DEP.' THIS WAS SAID WITH THE SOUND OF A GREAT DEAL OF ANNOYANCE IN HIS VOICE. IT WAS THEN THAT I CONCLUDED THAT WE HAD RESPONDED TO A TKOF CLRNC FOR FLT X2 EVEN THOUGH THE TWR CTLR HAD INCLUDED 'RWY 9L;' OUR RWY; IN THE TKOF CLRNC. JUST BEFORE WE SWITCHED TO DEP; WE HEARD THE FIRST CALL TO X2 WITH THE TWR CTLR SAYING 'X2 CLRED FOR TKOF RWY 17.' THIS WAS THE FIRST CALL WE HAD HEARD AND DEFINITELY ASSOCIATED WITH FLT X2. IT WAS THEN THAT MY SUSPICION WAS CONFIRMED THAT THERE HAD BEEN CONFUSION ALL AROUND. WE SWITCHED TO DEP CTL AND THE FLT PROCEEDED AND TERMINATED NORMALLY. INCIDENTALLY; FLT X2 SOON WAS BEING HANDLED BY THE SAME DEP CTLR AS WE WERE; AS IT WAS GOING IN THE SAME GENERAL DIRECTION AS WE WERE. CONTRIBUTING FACTORS INCLUDED CREW FATIGUE; THE USE OF 6 DIFFERENT FLT NUMBERS DURING OUR 8 LEGS THAT DAY; THE SHORTENING OF OUR FLT NUMBER BY PHL (NO OTHER FACILITY DOES THIS); BEING ON THE RWY AND EXPECTING TO HEAR A TKOF CLRNC; AND NOT KNOWING OR NOT HEARING ANOTHER COMPANY FLT IN THE TAXI PROCESS TO AN INTERESTING RWY. IT IS POSSIBLE THAT THE TWR CTLR MIXED THE FLT NUMBERS AND THE RWYS AND WAS SURPRISED TO SEE US (X1) ROLLING WHEN HE EXPECTED TO SEE X2 ROLLING. IT APPEARED PHL TWR COULD HAVE CLRED EITHER OF US TO TAKE OFF IN EITHER ORDER. BY WAY OF CORRECTIVE ACTIONS; I SHOULD HAVE IMMEDIATELY SPOKEN UP WHEN I HEARD AND RECOGNIZED X2. I BECAME COMPLACENT WHEN I ALSO HEARD 'RWY 9L' IN THE TKOF CLRNC. FURTHER; WHEN THE CTLR WAS 'SPITTING AND SPUTTERING;' AND; GIVEN MY UNEASINESS; I SHOULD HAVE ABORTED THE TKOF. WE WERE WELL BELOW V1 WHEN THIS HAPPENED. OTHER FACTORS INCLUDE THE FACT THAT THIS WAS MY SECOND DAY ON LINE; HAVING FINISHED MY IOE 5 DAYS EARLIER IN FLORIDA. I HAD MY HANDS FULL; AND THE CAPT WAS PROBABLY PAYING ATTN TO ME IN ADDITION TO HANDLING HIS OWN DUTIES. NOTHING LIKE THIS HAD EVER HAPPENED TO ME BEFORE IN MY 23 PREVIOUS YRS OF FLYING LIGHT GA ACFT OR IN GIVING FLT INSTRUCTION. MOST OF THAT EXPERIENCE WAS NOT IN BUSY TERMINALS; AND NEVER DID I SPEND 12+ HRS EITHER FLYING; PREPARING TO FLY; OR WAITING TO FLY. THIS IS COMMON WITH ACR OPS. I WILL BE FAR MORE COGNIZANT OF THE COMS HENCEFORTH; AND QUESTION ANYTHING THAT IS NOT PERCEIVED BY ME AS BEING 100% ACCURATE AND UNDERSTOOD BY BOTH CREW MEMBERS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR ADVISED THAT THEY HAVE AS LITTLE AS A 10 MIN TURNAROUND; AND THE PREVIOUS INBOUND CLRNC HAS USUALLY NEVER BEEN DROPPED FROM THE ARTCC HOST FLT PLAN SYS IN THAT SHORT OF TIME. THE COMPANY AND THE FAA HAVE DEVISED A SYS TO ALWAYS HAVE THE OUTBOUND CLRNC READY. THE COMPANY WILL USE THE SAME CALL SIGN THROUGHOUT THE DAY; UP TO A 10 TRIP DAY; ALTERNATING BTWN THE 4-NUMBER AND 3-NUMBER CALL SIGN. ON THIS OCCASION; THE FO DID NOT CHALLENGE THE CAPT WHEN THE FO THOUGHT THE WRONG CALL SIGN WAS ACKNOWLEDGED. SINCE THIS INCIDENT; THE RPTR ADVISED HE HAS BECOME MORE ASSERTIVE WITH THE PIC.