TRAINEE INSTRUCTOR PLT RESPONDED TO THE WRONG SIMILAR SOUNDING CALL SIGN AND TAXIED ONTO RWY FOR TKOF WHEN ANOTHER ACFT WAS CLRED FOR LNDG. ATCT LCL CTLR INTERVENED TO INSTRUCT RPTR INSTRUCTOR PLT TO CLR THE RWY FOR LNDG TFC.

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WE WERE CLRED 'INTO POS AND HOLD' ON RWY 29. WE DECLINED AND REQUESTED TO 'HOLD SHORT' AS WE NEEDED TO HAVE CFI APPLICANT (STUDENT) DEMONSTRATE A SOFT FIELD TKOF FROM A ROLLING TKOF. 1 ACFT LANDED AS WE HELD SHORT AS WE DISCUSSED SOFT FIELD TKOF PROCS. CFI APPLICANT ACKNOWLEDGED WHAT SHE WAS SURE WAS A CLRNC 'FOR RWY 15 CLRED FOR IMMEDIATE TO THE NW' (WE HAD REQUESTED A NW DEP INITIALLY). I HEARD THE TWR USE THE NUMBERS WHICH WAS VERY POSSIBLY A SARATOGA CALL SIGN WHICH WAS BEING CTLED BY HANSCOM TWR AT THE TIME. CFI APPLICANT ACKNOWLEDGED 'CESSNA CALL SIGN ROGER.' CFI APPLICANT TAXIED ONTO ACTIVE RWY 29 AFTER I HAD DETERMINED THERE WAS NO CONFLICT. THERE WAS AN ACFT; POSSIBLY A SARATOGA (ACFT AND COLOR PIPER SINGLE ENG ACFT) ON FINAL WHICH WE ESTIMATED TO BE BEYOND A MI OUT ON FINAL WITH LNDG LIGHTS ON. TWR CTLR XMITTED THAT WE NEEDED TO BE CLRED FOR TKOF; AND TO MAKE AN IMMEDIATE 180 DEGS AND HOLD SHORT. I TOOK CTL AT THIS POINT (WHEN TWR CALLED) AND EXPEDITED OUR DEP FROM THE RWY PAST THE HOLD SHORT LINE. I CALLED TWR ON VHF TO APOLOGIZE BEFORE TKOF AND AFTER THE MISSION ON GND. MY STUDENT AND I THEN WENT TO THE TWR AFTER SECURING THE ACFT AND APOLOGIZED TO THE TWR CHIEF; SENIOR CTLR AND TWR CTLR AND TO DISCUSS THE CIRCUMSTANCES. WE THEN DISCOVERED THAT THERE WAS AN ACFT WITH A SIMILAR CALL SIGN. THIS FLT WAS A TRAINING FLT FOR A CFI TRAINEE.