A B737-300 CREW; TAXIING FOR TKOF AT PHX; RESPONDED TO ANOTHER ACFT'S CLRNC; TAXIING INTO POS.

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WE PUSHED BACK OFF IN PHX ON TIME. TAXIED TO SPOT X AND CALLED GND FOR TAXI CLRNC. WE WERE CLRED TO TAXI TO RWY 25R. WE COMPLETED ALL THE CHKLISTS AND PULLED UP #1 AT RWY 25R. ANOTHER COMPANY WAS BEHIND AND OFF TO THE L OF US AND A COMPANY Y FLT WAS TAXIING DOWN TXWY E TOWARDS RWY 25R. NEITHER ACFT HAD ACCESS TO THE RWY. WE SAT AT THE END FOR A COUPLE OF MINS AND GOT IMPATIENT SO THE FO CALLED TWR 'READY.' THE FO'S XMISSION PARTIALLY BLOCKED; TWR'S XMISSION 'POS AND HOLD' (THE CALL SIGN WAS BLOCKED). WE ASSUMED THAT THE CLRNC INTO POS WAS OURS AND READ BACK THE CLRNC TO TWR. TWR DID NOT CATCH OUR ERROR WHEN WE READ IT BACK. ONCE IN POS; TWR ASKED 'WHO IS IN POS ON RWY 25R?' WE ANSWERED; AND WERE CHASTISED BECAUSE TWR SAID HE HAD CLRED COMPANY Y INTO POS AND HOLD. NO ACFT WERE ON FINAL; AND WE TOOK OFF UNEVENTFULLY. WE WERE NOT IN A HURRY DURING TAXI OUT. THIS SIT BOILS DOWN TO THE FACT THAT I ASSUMED THE CLRNC WAS FOR US; INSTEAD OF VERIFYING WITH TWR BEFORE TAXIING INTO POS AND HOLD. I USUALLY VERIFY EVERYTHING WHEN IN DOUBT; BUT IN THIS CASE; A LITTLE LAZINESS CREPT IN AND WE WERE CAUGHT IN THE ACFT OF 'ASSUMING.' SUPPLEMENTAL INFO FROM ACN 538031: WE SWITCHED TO TWR AND WAITED FOR AN ARR TO LAND. THE TWR CALLED WITH A CALL SIGN THAT ENDED IN 'Z;' THE FIRST PART WAS GARBLED; AND I READ BACK TO THE TWR; 'COMPANY FLT NUMBER INTO POS AND HOLD RWY 25R.' WE THEN TAXIED INTO POS AND HOLD ON RWY 25R. AFTER A SHORT DELAY THE TWR CLRED A COMPANY Y FLT; WHOSE CALL SIGN ENDED IN 'Z' FOR TKOF ON RWY 25R. COMPANY Y RESPONDED THAT THEY WERE HOLDING SHORT OF THE RWY. THE TWR THEN ASKED 'WHO WAS ON THE RWY?' I RESPONDED WITH OUR CALL SIGN AND ASKED IF THEY WOULD LIKE US TO TAXI CLR. HE THEN CLRED US FOR TKOF.