ACR MLG IN AN UNAUTH TAXI INCIDENT. NIGHT OP.

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AT XA57Z I DIRECTED THE FO TO CALL FOR TAXI CLRNC. THE CTLR WAS WORKING GND AND CLRNC DELIVERY FREQS SIMULTANEOUSLY. APPARENTLY; HE WAS AT THE TAIL END OF AN EXCHANGE OF INFO WITH AN ACFT ON CLRNC DELIVERY THAT WE COULD NOT HEAR. WE INTERPRETED THE CTLR'S LAST 2 XMISSIONS TO THE OTHER ACFT AS OUR CLRNC TO TAXI. WHAT WE HEARD AFTER REQUESTING TAXI CLRNC WAS 'HOW LONG WILL IT TAKE YOU TO BE READY FOR TKOF?' THE FO; ESTIMATING HOW LONG IT WOULD TAKE TO TAXI TO RWY 7; SAID; 'ABOUT 5 MIN.' THE CTLR THEN SAID; 'THANKS FOR THE HELP;' OR SOMETHING TO THAT AFFECT; AND WE BEGAN OUR TAXI. AS WE APCHED RWY 15L ON TXWY A; I ASKED THE FO IF THE CTLR HAD ACTUALLY SAID; '...TAXI TO RWY 7?' THE LCL CTLR APPARENTLY SAW OUR MOVEMENT AND; AFTER HAVING CLRED A SINGLE-ENG AIRPLANE FOR TKOF ON RWY 15R; DIRECTED THE GND CTLR TO INSTRUCT US TO HOLD SHORT OF RWY 15R. I HEARD THIS HOLD SHORT CLRNC JUST AS WE BEGAN TO CROSS RWY 15L; SO I IMMEDIATELY STOPPED THE AIRPLANE. THE ABRUPT STOP CAUSED A MALE FLT ATTENDANT DOING THE PAX DEMONSTRATION; TO MOVE BACKWARDS INTO THE EDGE OF THE FIRST CLASS/COACH CABIN DIVIDER WALL. HE STAYED ON HIS FEET AND DID NOT FEEL ANY DISCOMFORT UPON ARR AT THE DEST. I CALLED THE CTLR FROM OUR DEST TO DISCUSS THE INCIDENT AND DETERMINE IF WE HAD INDEED TAXIED WITHOUT A CLRNC. THE CTLR WAS NOT 100 PERCENT CERTAIN BUT SAID THAT IT DID NOT CAUSE A TFC CONFLICT OR RWY INCURSION. THIS INCIDENT WAS THE RESULT OF INITIATING TAXI WITHOUT HEARING THE WORDS; 'TAXI TO RWY 7.' THE PRACTICE OF ATC CONSOLIDATING POS USING MORE THAN 1 FREQ WAS A CONTRIBUTING FACTOR. IF GND CTL AND CLRNC DELIVERY WERE COMBINED ON 1 FREQ; IT WOULD HAVE BEEN MORE READILY APPARENT THAT THE CTLR'S XMISSIONS WERE INTENDED FOR ANOTHER ACFT.