ATCT LCL CTLR GIVES A TKOF CLRNC TO THE FLT CREW OF A B737 AFTER IT IS ON A TKOF ROLL ON RWY 25L AT PHX; AZ.

Narrative

OUR DAY STARTED WITH AN XA30 LOBBY IN CLE AND DAY #2 OF AN IOE. THE ARR INTO STL WAS THROUGH A SIGNIFICANT AREA OF TSTMS AND SOMEWHAT CHALLENGING. OUR DEP OUT OF STL WAS EQUALLY CHALLENGING. THE FO'S NEWNESS WITH CIVILIAN OPS AND A NEW EAR PIECE WAS GIVING RISE TO SOME CONFUSING RADIO CALLS. I STARTED MAKING SOME OF THE RADIO CALLS TO EXPEDITE THE PROCESS. OUR THIRD LEG WAS PHX TO LAS. WE PUSHED BACK FROM GATE. WE HELD FOR WHAT MUST HAVE BEEN AT LEAST 10-15 MINS. I MADE A PA TO THE PAX USING THE FIRST OBSERVER'S HAND SET AND THE #3 RADIO. I'D BEEN USING THIS RADIO FOR PA'S ALL MORNING. MUCH TO MY SURPRISE; THIS RADIO HAD BEEN CHANGED FROM PA TO VHF #1; SO MY PA WAS DONE ON A RATHER CONGESTED GND CTL FREQ. WE SLOWLY WORKED OUR WAY S OF TXWY R WHEN A FLT ATTENDANT CALLED TO LET ME KNOW A PAX HAD TO MAKE AN EMER LAVATORY RUN. I SAID YES; JUST LEAVE THE COCKPIT DOOR OPEN. WE HELD OUR POS FOR SEVERAL MINS; THEN RESECURED THE CABIN AND CALLED READY. I THEN CONTINUED TO TAXI TOWARD TXWY E11 TO HOLD SHORT OF RWY 25L AND BE READY TO GO. AS WE APCHED THE HOLD SHORT LINE; WE WERE CLRED ON TO HOLD RWY 25L. DURING THIS TIME; I THOUGHT TWR TOLD US WE WERE CLRED TO TAKE OFF. IT WAS THE FO'S DEP AND I REMINDED HIM ABOUT SMOOTH PWR APPLICATION AS WE LINED UP ON CTRLINE. AS I ADVANCED THE THROTTLES; I GOT A QUESTIONING LOOK FROM THE FO. I SAID 'WE'VE BEEN CLRED FOR TKOF.' HE'D HAD A LOT OF DIFFICULTY WITH HIS HEADSET EAR PIECE BEING ABLE TO HEAR CLRLY THROUGHOUT THE DAY; AND I BELIEVED THIS WAS ANOTHER OCCASION OF NOT BEING ABLE TO HEAR. APPROX 1200 FT DOWN THE RWY; PHX TWR SAID 'ACR X; YOU'RE CLRED FOR TKOF.' HIS TONAL INFLECTION INDICATED MY ERROR. WHEN SWITCHING TO DEP; HIS PARTING COMMENT WAS 'HAVE A GOOD DAY.' I THINK HE KNEW THAT I NOW KNEW MY ERROR. I'VE ALWAYS THOUGHT THIS WOULD BE AN EASY AND GRIEVOUS ERROR TO MAKE. I ALSO THOUGHT I PAID ATTN TO THIS AREA OF CONCERN AND YET HAD JUST DONE IT.