A B757 CREW; HOLDING IN POS AT MIA; ACCEPTED A CLRNC THEY THOUGHT WAS INTENDED FOR THEM. HOWEVER; AN ALERT COMPANY JUMP SEATER ADVISED THE CLRNC WAS FOR ANOTHER ACFT.

Narrative

MIA TWR WAS OPERATING BOTH RWY 27L&R TKOFS AND LNDGS WHILE WE WERE SITTING ON THE ACTIVE RWY; 27R WAITING TO TAKE OFF; IN POS AND HOLD. THERE WERE 4 PEOPLE IN THE COCKPIT -- FO; FAA INSPECTOR; NON REVENUE PLT FOR ACR X; AND MYSELF. WE WERE TOLD BY MIA TWR THAT WE WOULD BE ON THE RWY FOR ABOUT 2 MINS WAITING FOR A HELI TO LAND OPPOSITE DIRECTION ON RWY 9L. AS USUAL; THERE WAS A CONSIDERABLE AMOUNT OF RADIO COMMUNICATIONS WITH THE MIA TWR WORKING BOTH RWYS. THE FO; FAA INSPECTOR AND MYSELF HEARD THAT WE WERE CLRED FOR TKOF ON RWY 27R; ONCE THE HELI CLRED THE RWY. I READ BACK TO THE CTLR; 'ACR X CLRED FOR TKOF ON RWY 27R; TURN R TO HDG 290 DEGS.' THE NON REVENUE PLT QUESTIONED WHETHER WE WERE CLRED; OR ACFT Y ON RWY 27L; WAS CLRED FOR TKOF. I QUESTIONED THE CLRNC WITH MIA TWR AND THE NON REVENUE PLT WAS RIGHT -- THE TKOF CLRNC WAS FOR ACFT Y. THE TWR CTLR NEVER SAID ANYTHING ABOUT SIMILAR SOUNDING CALL SIGNS AND GAVE THE EXACT SAME CLRNC TO ACFT Y. WE DID NOT TAKE OFF; BUT IT WAS ONLY BECAUSE THE NON REVENUE PLT QUESTIONED THE TKOF CLRNC. AS I ALREADY MENTIONED; EVEN THE FAA INSPECTOR THOUGHT WE WERE CLRED FOR TKOF. THE ONLY REASON THE NON REVENUE PLT HAD ANY QUESTION WAS BECAUSE HE HEARD ACFT Y GET A CLRNC TO TAXI INTO POS AND HOLD ON RWY 27L; JUST PRIOR TO THE CONFUSING TKOF CLRNC. THE FAA INSPECTOR FROM NYC SAID WE DID AN EXCELLENT JOB WITH GREAT CRM. HE SAID HE WOULD PURSUE THIS FURTHER WITH MIA TWR FOLKS.