2 B737'S FROM THE SAME ACR COMPANY WITH SIMILAR SOUNDING CALL SIGNS WERE PUSHED BACK FROM THEIR GATES AND TAXIED OUT FOR TKOF. THE FIRST B737 (XYZ) KEPT ANSWERING INSTRUCTIONS AND CLRNCS FOR THE SECOND B737(SYZ). WHEN THE FIRST ACFT WAS AT THE DEP END OF THE RWY HE TOOK A TKOF CLRNC FOR HIS COMPANY FLT AND AT THAT TIME HE BECAME AWARE OF THE CALL SIGN SIMILARITY WHEN ADVISED BY THAT ACFT. XYZ TAXIED OFF THE RWY AND SYZ TOOK OFF.

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LANDED AS ACR FLT XYZ OAK-ONT AND CLRED RWY 26R AT ONT. TAXIED TO THE GATE AS INSTRUCTED ON TWR FREQ. AFTER DEBOARDING AND BOARDING PAX IN ONT; CONTINUED AS ACR XYZ ONT-PHX. CALLED FOR PUSHBACK ON GND FREQ. GND RESPONDED; 'ACR SYZ PUSHBACK APPROVED.' I RESPONDED; 'UNDERSTAND ACR XYZ PUSH APPROVED?' GND RESPONDED; 'XYZ PUSH APPROVED.' MEANWHILE; ANOTHER COMPANY ACFT WAS PUSHING OFF OF ANOTHER GATE NEARBY; BUT HE WAS PUSHING BACK ON TWR FREQ INSTEAD OF GND. UNBEKNOWNST TO US; HIS CALL SIGN WAS ACR SYZ. WE TAXIED UNEVENTFULLY ON GND FREQ AHEAD OF THE OTHER ACFT ON GND FREQ. HE TAXIED ON TWR; UNBEKNOWNST TO US. TWR AND GND CTL WERE APPARENTLY THE SAME GUY. WE REACHED THE END OF RWY 26R READY FOR TKOF AND CALLED 'READY FOR TKOF ACR XYZ' ON TWR FREQ. TWR TOLD US TO TAXI FULL LENGTH AND HOLD SHORT. A SHORT TIME LATER; TWR CALLED 'ACR XYZ CLRED FOR TKOF' OR WHAT SOUNDED LIKE XYZ. THE OTHER COMPANY ACFT HAD NOT CHKED IN ON TWR YET. I RESPONDED 'ACR XYZ CLRED FOR TKOF.' WE PULLED ONTO THE RWY AND HEARD OVER THE RADIO; FROM THE OTHER COMPANY ACFT; 'I THINK HE SAID SYZ!' I CALLED TWR AND ASKED IF HE SAID SYZ OR XYZ. HE SAID 'ACR SYZ.' I TOLD HIM WE HAD RESPONDED TO THE ORIGINAL CALL AND NEEDED TO EXIT THE RWY. WE DID AND ACR SYZ TAXIED ON AND TOOK OFF UNEVENTFULLY. THERE WERE NO ACFT ON FINAL AND WE GOT OFF OF THE RWY IMMEDIATELY. WHAT CAUSED THIS PROB WAS: 1) TWR TALKING TO 2 DIFFERENT ACFT WITH VERY SIMILAR CALL SIGNS ON 2 DIFFERENT FREQS AND NOT INFORMING EITHER. BUT TWR MIGHT NOT HAVE KNOWN WE WERE ON DIFFERENT FREQS IF HE WAS CTLING BOTH ON SIMULCAST. 2) TWR CALLING US THE WRONG ACFT NUMBER ORIGINALLY AT PUSH LED US TO BELIEVE THEY THOUGHT OUR CALL SIGN WAS SOMETHING DIFFERENT. 3) NO CLARIFICATION FROM US OR TWR OR SYZ THAT IT WAS SYZ WHO WAS CLRED FOR TKOF RATHER THAN XYZ.