AFTER SWITCHING TO PIT TWR FREQ; A B737-300 CREW HEARD A TKOF CLRNC. AFTER VERIFYING CLRNC WHILE ON THE RWY; CREW REALIZED TKOF CLRNC WAS FOR ANOTHER ACFT.

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WE WERE ON TXWY B; ENRTE TO RWY 28R AT PIT. WE WERE #1 FOR THAT RWY; AND NO OTHER ACFT AROUND. WE COMPLETED THE BEFORE TKOF CHKLIST THEN SWITCHED TO TWR CTL AS WE APCHED THE END OF THE RWY. WE HEARD TWR SAY A CALL SIGN (WHICH WE THOUGHT WAS OURS) CLRED FOR TKOF. BECAUSE WE HAD JUST SWITCHED OVER; AND THE XMISSION SEEMED A LITTLE GARBLED; THE FO SAID CONFIRM COMPANY NUMBER CLRED FOR TKOF RWY 28R. THEN WE HEARD AN AFFIRMATIVE. WE CONTINUED AROUND THE CORNER INTO POS ON THE RWY; BUT DIDN'T QUITE HAVE A WARM AND FUZZY; SINCE WE DIDN'T HEAR THE CTLR REPEAT OUR CALL SIGN WHEN WE WERE CONFIRMING OUR TKOF CLRNC. SO WE STOPPED AT THE NORMAL POS AND HOLD AREA ON THE RWY; AND ASKED TWR PLEASE CONFIRM COMPANY NUMBER CLRED FOR TKOF. THEN THE CTLR STATED NEGATIVE; YOU ARE NOT EVEN CLRED TO BE ON THE RWY. WE TAXIED CLR IMMEDIATELY. WE EXITED THE RWY BACK ONTO TXWY B. THE EXIT WAS ONLY ABOUT 50 FT AWAY. WE CONTINUED BACK TO THE HOLD SHORT LINE WHERE WE WERE THEN CLRED FOR TKOF ALMOST IMMEDIATELY. THE TWR CTLR WAS APPARENTLY WORKING 2 SEPARATE FREQS AND HAD CLRED SOMEONE ON THE OTHER SIDE OF THE ARPT FOR TKOF. THE 'AFFIRMATIVE' THAT THE CTLR GAVE US WHEN WE ASKED HER TO CONFIRM WAS FOR ANOTHER ACFT ON THE OTHER FREQ. THIS WAS MY FIRST TIME IN PITTSBURGH AND MY FO'S SECOND AND FOR SOME REASON IT NEVER OCCURRED TO ME THAT SHE COULD HAVE BEEN TALKING TO SOMEONE ELSE ON THE OTHER SIDE OF THE ARPT ON ANOTHER FREQ. THE LESSON LEARNED OR ONE THAT I NORMALLY KNOW BETTER; IS THAT WE SHOULD NOT HAVE ACCEPTED THE 'AFFIRMATIVE' AS THE CONFIRMATION THAT SHE WAS TALKING TO US. WE SHOULD HAVE GOTTEN HER TO SAY OUR CALL SIGN AND REPEAT THE CLRNC BEFORE XING THE HOLD SHORT LINE. FORTUNATELY; FINAL WAS CLR AND THERE WERE NO OTHER ACFT AROUND AND NO CONFLICTS.