B727 CREW AT LUB INCORRECTLY CROSSED THE RWY THEY WERE CLRED TO.

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WE WERE A B727 TAXING FROM OUR RAMP ONTO TXWY S FOR TKOF ON RWY 17L. AS WE APCHED TXWY S THE FO CALLED GND FOR CLRNC TO TAXI FOR TKOF. GND REPLIED TAXI ON S FOR RWY 17R. I DIDNT QUITE HEAR THE INSTRUCTION SO I ASKED THE FO WHAT OUR CLRNC WAS. HE SAID TAXI TO RWY 17R. WE CLRED RWY 17R FOR TFC ON FINAL AND SEEING NO ONE ON APCH PROCEEDED TO CROSS RWY 17R ON TXWY S. AFTER CROSSING THE RWY GND RADIOED THAT HE WANTED US TO HOLD ON S PRIOR TO CROSSING RWY 17R BECAUSE HE INDEED DID HAVE TFC ON FINAL. HE THEN REPEATED HIS INSTRUCTIONS TO US THAT WOULD INDICATE ACCORDING TO FAA STANDARD PHRASEOLOGY THAT WE WERE CLRED TO CROSS ALL RWYS UP TO BUT NOT INCLUDING THE DEPARTURE RWY. HE APOLOGIZED TO US FOR THE CONFUSION IN HIS POSSIBLE WORDING AND WE APOLOGIZED TO HIM FOR NOT BEING MORE ALERT TO HIS XMISSION. THE LIGHT TWIN ON FINAL WAS IN NO JEOPARDY AND LANDED WITHOUT INCIDENT. SUPPLEMENTAL INFO FROM ACN 529129: CLRED TAXI TO RWY 17R; VIA TXWY S. WE UNDERSTOOD CLRED TO CROSS 17R AT S WHICH IS THE ONLY TXWY OUT OF CARGO RAMP TO THE RWY. TAXI RTE REQUIRES CROSSING RWY 17 AT S AND THEN N ON PARTIAL TXWY TO APCH END. AS WE CLRED THE RWY GND TOLD US TO CONTINUE BUT THAT WE SHOULD HAVE HELD SHORT ON RWY 17 AT S. WE ALL CONCURRED THAT WE WERE CLRED TO CROSS PRIOR TO CROSSING AND THAT BOTH DIRECTIONS WERE CLR VISUALLY.