CAPT OF A MLG FAILED TO STOP AT THE HOLD SHORT LINE DURING TAXI FOR TKOF DUE TO A MISUNDERSTANDING WITH TWR INSTRUCTIONS.

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OUR ACFT WAS #1 FOR TKOF ON RWY 12R; BOTH PLTS HEARD AND FO ACKNOWLEDGED TO STL TWR THAT WE WERE CLRED INTO POS FOR TKOF. AS THE ACFT WAS TURNED THE FO CONFIRMED TO THE STL TWR TO THE EARLY ACKNOWLEDGEMENT; THE STL TWR MADE NO RESPONSE. AS THE NOSE OF ACFT TURNED TOWARD THE RWY; WE SAW A LIGHT TWIN IN POS. AGAIN THE FO CALLED THE TWR; THIS TIME THE STL TWR RESPONDED BY SAYING THAT WE WERE NOT CLRED INTO POS; AND HE THANKED US FOR OUR XMISSION. A 727 WAS ASKED TO GAR BECAUSE THE NOSE OF OUR ACFT WAS OVER THE HOLD SHORT LINE FOR RWY 12R. WE WERE THEN CLRED FOR TKOF. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THE ACFT OPERATED IN THIS INCIDENT WAS A DC-9. HE FURTHER STATED THAT HE FELT THAT THE TWR MUST HAVE BEEN BLOCKED BY ANOTHER ACFT WHEN IT APPEARED TO NOT ANSWER THEIR CONFIRMATION CALL FOR TAXI INTO POS AND HOLD. HE STATED THAT HE IS ALWAYS READY FOR TKOF WHEN ISSUED THAT INSTRUCTION SO HAS NO PROB IN COMPLYING WITH AN EXPECTED EXPEDITED TKOF. HE MENTIONED THAT HE HAD ANOTHER INSTANCE SUBSEQUENTLY; WHEREIN HE THOUGHT THEY WERE CLRED INTO POS SINCE THEY WERE THE FIRST ONE AT THE SIDE OF THE RWY. HOWEVER; IT TURNED OUT THAT ANOTHER ACFT ON THE OTHER SIDE OF RWY AT AN INTXN WAS #1 AND CLRED INSTEAD OF HIM. IT DOES PAY TO BE ALERT AND READY TO GO!