MD80 EXPERIENCES ATC DIRECTED TKOF ABORT AT MIA.

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ON A SHORT TAXI TO RWY 27R AT MIA; WE WERE TOLD TO BE READY FOR AN IMMEDIATE DEP; SO I MADE AN ANNOUNCEMENT TO THE FLT ATTENDANTS. I HAD JUST GOT A NEW BOOM MIKE; AND THE EAR PIECE WAS SOMEWHAT LOOSE. I HEARD TWR ISSUE DEP CLRNC WITH THE CALL SIGN ENDING WITH XX. THE COPLT READ IT BACK; AND I ADDED THE PWR TO TAKE THE RWY AND TAKE OFF. SOMETHING IN THE OVERLAPPING COMS BOTHERED ME; THOUGH; BECAUSE I REALIZED I HAD HEARD A #9. SO I ASKED THE COPLT IF HE WAS SURE NOW THAT THIS WAS FOR US. HE SAID IT WAS -- RATHER THAN VERIFYING IT; WHICH IS WHAT I REALLY WANTED HIM TO DO. I RATIONALIZED THE #9 BEING A MISTAKEN RWY NUMBER FOR A SPLIT SECOND; BUT I REALIZED ALSO; BY THIS TIME; I DID NOT HEAR THE WHOLE CLRNC FOR MYSELF! I LOOKED DOWN THE RWY AND HEARD TWR CALL FOR OUR ABORT; WHICH WE DID IMMEDIATELY BELOW AIRSPD INDICATION AND TAXIED OFF WITHIN THE FIRST 1000 FT OR SO. AS WE CLRED THE RWY; WE HEARD ANOTHER PLT COME ON THE AIR AND STATE 'THAT'S A RWY INCURSION.' WE TALKED TO THE TWR ON ANOTHER FREQ; AND THEY SAID THAT THEY HAD CLRED ANOTHER ACFT TO CROSS AT THE FAR END OF THE FIELD; BUT HE HAD NOT CROSSED THE LINE BEFORE WE HAD GOTTEN OFF; AND THAT THE ORIGINAL CLRNC WAS FOR ACFT ACR Y ON RWY 27L. MY COPLT'S RESPONSE WAS BLOCKED EXCEPT FOR THE #XX; WHICH TWR HEARD AT THE END OF ACR Y'S RESPONSE TO ITS CLRNC. THE 2 COMS TOTALLY OVERLAPPED. TWR ALSO STATED THAT THE CAPT FROM FLT ACR Z WAS THE PERSON WHO KEPT INTERRUPTING THEM WITH LEGALITIES OVER AN INCURSION BEFORE THEY COULD EVEN REVIEW THE TAPES. SUPPLEMENTAL INFO FROM ACN 583450: SHORTLY AFTER THE B757 WAS AIRBORNE; I HEARD ACR X CLRED FOR IMMEDIATE TKOF NO DELAY. I RESPONDED 'ACFT X CLRED FOR TKOF;' AND DID NOT HEAR ANYTHING ELSE FROM THE TWR. PWR WAS SET WHEN AT THE SAME TIME; WE SAW ANOTHER ACFT AT THE END OF THE RWY BEGINNING TO CROSS RWY 27R; AND TWR CALLING FOR US TO ABORT. TWR WAS VERY QUICK TO CATCH THE CONFUSION; AND CALLED FOR THE IMMEDIATE ABORT.