AN MD80 CREW; TAXIING FOR TKOF AT ATL; TAXIED INTO POS AS THEY WERE CLRED BY TWR. HOWEVER; TWR CLRED ANOTHER ACFT TO LAND SINCE NO ACKNOWLEDGEMENT WAS HEARD FROM FLC. FLC HAD MISTAKENLY ACKNOWLEDGED ATC CLRNC ON ACFT CABIN INTERCOM CHANNEL.

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DURING TAXI TO RWY 27R ON TXWY M PRIOR TO TXWY D; GND CTL SWITCHED US TO TWR FREQ. TWR IMMEDIATELY CLRED US TO 'POS AND HOLD ON RWY 27R.' THE FO ACKNOWLEDGED THE CALL. AS WE TURNED THE CORNER TO TAXI INTO POS AND HOLD; TWR TOLD US TO 'HOLD SHORT OF RWY 27R.' THE FO RESPONDED THAT WE WERE ALREADY ON THE RWY. TWR SAID WE HAD NOT ACKNOWLEDGED HIS POS AND HOLD CALL; SO HE HAD CLRED ANOTHER ACFT TO LAND ON RWY 27R. THE FO SAID WE HAD ACKNOWLEDGED HIS CALL. TWR SAID WE WERE CLRED FOR AN IMMEDIATE TKOF. AT THE TIME TWR CALLED US TO POS AND HOLD; THE FO WAS ACCOMPLISHING THE AFTER START AND BEFORE START CHKLISTS. HE WAS ACTUALLY TALKING TO THE FLT ATTENDANTS TO SEE IF THEY WERE READY FOR TKOF WHEN TWR CALLED. I ALSO HAD MY CABIN/SVC INTERPHONE SELECTED; LISTENING TO THE FLT ATTENDANTS AND FO. WHEN HE RESPONDED TO TWR; THE FEEDBACK I HEARD WAS APPARENTLY ON THAT SYS AND NOT ON TWR FREQ AS I THOUGHT. THE FLT ATTENDANTS LATER SAID THEY HEARD HIS RESPONSE TO THE TWR ON THE CABIN/SVC INTERPHONE. THE FO HAD NOT RESELECTED THE #1 RADIO WHEN RESPONDING TO THE TWR CALL. IT WAS OUR MISTAKE NOT RESPONDING TO TWR ON THE CORRECT RADIO; BUT HE SHOULD HAVE INQUIRED AGAIN TO SEE IF WE HAD HEARD HIM CLR US INTO POS AND HOLD. ESPECIALLY SINCE WE WERE OVER 6000 FT FROM THE END OF THE RWY WHEN HE CLRED US INTO POS AND HOLD ON RWY 27R IN ATL. SUPPLEMENTAL INFO FROM ACN 540323: THIS IS A BUSY TIME FOR FLC'S. WE ARE REGULARLY RUNNING DELAYED START; BEFORE TKOF; SWITCHING FROM GND TO TWR; TALKING WITH OUR FLT ATTENDANTS AND RECEIVING THEIR BEFORE TKOF RPT. A RWY INCURSION IS TOO BIG OF A THREAT FOR TWR NOT TO ASK AGAIN; REISSUE CLRNC; OR RESCIND CLRNC; WHEN THEY DO NOT HEAR A RESPONSE TO A CLRED POS AND HOLD CLRNC. ESPECIALLY WHEN ISSUED AT AN UNUSUAL POS; IE; 2000 FT UP TXWY M.