FLC OF AN MDT ACR ACFT INADVERTENTLY CROSSED AN ACTIVE RWY WITHOUT CLRNC DURING TAXI FOR TKOF.

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I CALLED FOR TAXI CLRNC FROM GATE AT MIA. GND CTL CLRED US TO TAXI TO RWY 9L. APCHING THE FIRST TXWY ACCESSING THE OUTER (WE WERE ON THE INNER) GND CTL TOLD US TO JOIN THE OUTER AND TAXI TO RWY 27L. I QUESTIONED THIS CLRNC BECAUSE THE ARPT WAS IN AN E OP; WE WERE NOT TOLD TO EXPECT A W RWY AND THE INSTRUCTIONS CAME FROM A DIFFERENT VOICE. THE CTLR REPEATED THE INSTRUCTIONS AND WE TAXIED TO RWY 27L. HOLDING SHORT OF RWY 27L THE CTLR ASKED IF WE COULD ACCEPT AN INTXN TKOF ON RWY 30. AFTER THE CAPT AND I DISCUSSED THIS; WE ANSWERED THE CTLR IN THE AFFIRMATIVE. AFTER WAITING ABOUT 1 MIN; THE CTLR TOLD US TO TAXI UP TO BUT HOLD SHORT OF RWY 30. AFTER READING BACK HIS CLRNC TO HOLD SHORT OF RWY 30; I MADE MY PA TO THE CABIN WHILE THE CAPT TAXIED ACROSS RWY 27L TO RWY 30. JUST AS WE WERE ABOUT TO CLR RWY 27L ON THE OTHER SIDE; I LOOKED DOWN THE RWY AND SAW AN ACFT ON THE RWY BEGINNING TO TURN OFF. AT THIS TIME THE GND CTLR TOLD US WE WERE NOT CLRED TO CROSS RWY 27L; THAT THERE WAS AN ACFT TOUCHING DOWN AND THAT WE WERE EXPECTED TO HAVE MADE A 180 DEG TURN ON THE OUTER AND TAXI TO RWY 30 TO HOLD SHORT. HE ALSO ASKED US TO CLR RWY 27L. AT THIS TIME WE WERE BTWN RWY 27L AND RWY 30. CONTRIBUTING TO THIS EVENT IS THAT THE CTLR SPOKE FASTER THAN I'VE EVER HEARD ONE SPEAK. THE FREQ WAS NOT CONGESTED; BUT HE COULD HAVE BEEN WORKING 2 FREQS. ALSO; THE INSTRUCTIONS CAME TO US FROM 2 VOICES -- INDICATING ONE OF THEM WAS PROBABLY BEING SUPERVISED. ALSO; I SHOULD HAVE ASKED HIM TO BE MORE SPECIFIC WHEN HE ASKED US IF WE COULD ACCEPT AN INTXN TKOF; NOT KNOWING WHAT INTXN HE WAS REFERRING TO. AFTER DISCUSSING THIS WITH THE CAPT; HE ALSO SAID HE DID NOT HEAR THE CTLR TELL US TO TURN AROUND AND TAXI TO RWY 30 ON THE OUTER. I HEARD AND REPEATED THE IMPORTANT PART -- TO HOLD SHORT OF RWY 30; AND GIVEN A CLRNC TO TAXI TO A RWY MEANS YOU CAN CROSS RWYS AND TXWYS TO DO IT. I ASSUMED THAT IS WHAT HE EXPECTED AND THAT THE INTXN TKOF PLAN HE HAD WAS CANCELLED. NEXT TIME I WILL ASK FOR SPECIFIC INSTRUCTIONS TO BE GIVEN TO ME IN A SLOWER VOICE.