RWY INCURSION BY A B737-700 ON RWY 33 AT ISP; NY.

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PUSHED OFF GATE; TAIL FACING E. FINISHED THE CHKLIST AND CALLED FOR TAXI. IT WAS A SHORT TAXI TO RWY 33L. I WAS TAXIING REAL SLOW WHILE WAITING FOR THE FLT ATTENDANTS TO FINISH BRIEFING THE PAX. ONCE BRIEFING WAS COMPLETED; I PUSHED THE FLT ATTENDANT CALL BUTTON TO INFORM THEM WE WERE READY FOR TKOF. I TURNED UP THE VOLUME TO HEAR THE FLT ATTENDANT RESPOND. AFTER RETURNING BACK TO COM #1; I CAUGHT THE TAIL END OF THE CONVERSATION BTWN THE FO AND TWR. I ASKED THE FO WHAT WAS GOING ON. HIS RESPONSE WAS TO TAXI INTO POS AND HOLD. I REPEATED BACK WHAT HE SAID. WE SAT ON THE RWY FOR ABOUT 1/2 TO 2 MINS. THE TWR CALLED US AND SAID THAT HE TOLD US TO HOLD SHORT OF RWY 33L. I CALLED THE TWR IN ISP AFTER ARRIVING INTO BWI. THE TWR STATED THAT THERE WERE NO PROBS WITH ANY OTHER ACFT; AND DON'T WORRY ABOUT A THING JUST BTWN US (I HOPE). THE TWR STATED THAT HE WAS TIED UP TRYING TO FIND A POLICE HELI. I ASKED WOULD THERE BE ANY PROB LISTENING TO THE TAPE. HE STATED 'NO.' THE TAPE REVEALED THAT THE TWR HAD SAID TO HOLD SHORT OF RWY 33L. THE FO RESPONDED BACK 'IN POS AND HOLD ON RWY 33L.' SO; THE TWR FAILED TO RESPOND BACK TO WHAT THE FO SAID. THE FO FAILED TO READ BACK THE RIGHT CLRNC INSTRUCTION. WHAT CAN BE DONE TO PREVENT THIS FROM HAPPENING. I GUESS DON'T MOVE THE ACFT UNTIL BOTH OF US ARE MONITORING COM #1. SUPPLEMENTAL INFO FROM ACN 579899: I (PNF) CONTACTED TWR STATING WE WERE READY FOR DEP. HE STATED 'TAXI IN POS AND HOLD RWY 33L.' AFTER RECEIVING CLRNC WE PERFORMED BELOW THE LINE AND TAXIED INTO POS AND HOLD. WE WERE ON THE RWY FOR ABOUT 20 SECONDS BEFORE THE TWR SAID 'COMPANY NUMBER; I DIDN'T CLR YOU IN POS. YOU ARE NOW CLRED FOR TKOF.' THERE WAS SOME STATIC ON THE FREQ THAT COULD HAVE CAUSED SOME QUESTION ON THE RADIO; BUT HIS CLRNC WAS CLR IN '...POS AND HOLD...' AND WE WERE ON THE RWY FOR 20 SECONDS BEFORE HE SAID ANYTHING. IN THE FUTURE IF THERE IS ANY DOUBT WITH THE CLRNC OR CLARITY I WILL ASK FOR ADDITIONAL VERIFICATION.