ALTDEV ALT EXCURSION DUE TO REPLY TO WRONG CALL SIGN INCOMPLETE CALL SIGN. SIMILAR A/N.

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IFR FLT OMN-AUS. THIS LEG WAS OMN 0R9 (HAMMOND MUNICIPAL) FOR A FUEL STOP. TRAVELING W ON V-70 WE WERE APPROX 5 MI N OF COURSE DUE TO A DEV AROUND A CUMULUS BUILDUP; WE WERE CLRED DIRECT HAMMOND VOR. THEN WE WERE GIVEN VECTORS TO INTERCEPT V-70. THE REASON FOR THIS APPARENTLY WAS FOR SPACING FROM ACFT Y WHO WAS INBOUND TO 0R9. THE PROB OCCURRED BECAUSE OUR CALL SIGN WAS ABXYZ AND THE OTHER ACFT'S WAS CDXYZ. WE WERE AT 5000 FT MSL APPROX 20 MI OUT FROM HAMMOND ON THE VECTOR WHEN I REQUESTED A DSCNT TO 3000 FT. A FEW SECONDS LATER THE CTLR CLRED XYZ TO 3000 FT AND I ACKNOWLEDGED THE CLRNC OUT OF 5000 FT FOR 3000 FT. APPARENTLY THE DSCNT WAS MEANT FOR ACFT Y (WHO WAS NOT DSNDING); ALTHOUGH HE MUST HAVE THOUGHT IT WAS FOR ME (AS I DID) OR HE WOULD HAVE ACKNOWLEDGED THE DSCNT CLRNC. WHILE I WAS DSNDING THROUGH ABOUT 4300 FT MSL THE CTLR CALLED AND SAID WE WERE SUPPOSED TO BE AT 5000 FT. HE ISSUED US AN IMMEDIATE CLB TO 5000 FT (AND A VECTOR; I THINK) AND AN IMMEDIATE DSCNT TO 3000 FT FOR THE OTHER ACFT. WE NEVER SAW THE OTHER ACFT BUT I DON'T THINK WE CAME WITHIN 3-4 MI OF ITS POS DUE TO IFR SEPARATION. THE REMAINDER OF THE FLT WAS UNEVENTFUL. THE CTLR USED BOTH ACFT'S FULL CALL SIGNS FROM THIS POINT ON; AND EMPHASIZED THE FIRST 2 NUMBERS TO ASSURE THERE WAS NO DOUBT WHO HIS INSTRUCTIONS WERE FOR. I SPOKE WITH THE PLT OF ACFT Y ON THE GND AT HAMMOND AND DISCOVERED HE WAS CONTINUING W ON THE SAME RTE AS I WAS PLANNING; SO I WAITED ABOUT A HALF HR AFTER HE DEPARTED BEFORE I LEFT SO A SIMILAR PROB WOULDN'T OCCUR. I THINK THE PROB OCCURRED BECAUSE YOU BECOME ATTUNED TO HEARING YOUR CALL (XYZ IN THIS CASE) AND FOCUS YOUR ATTN ON IT; ANY DISTR AND YOU CAN MISS THE FIRST PART OF THE CALL SIGN BECAUSE BY HABIT YOUR EARS AREN'T TRAINED TO LISTEN TO ANYTHING BUT THE LAST 3 NUMBERS (LETTERS). I DON'T REMEMBER IF THE CTLR WAS USING OUR FULL CALL SIGNS AT THIS POINT. SINCE I HAD REQUESTED THE DSCNT TO 3000 FT; WHEN THE CTLR CLRED 'XYZ' TO 3000 FT I THOUGHT IT WAS MY CLRNC. ONE INACTION ON MY PART I REGRET WAS IN NOT QUERYING THE CTLR ABOUT THE DSCNT. HE GAVE US A NEW VECTOR AS WE WERE DSNDING; AND AS I ACKNOWLEDGED THE TURN; I WANTED TO ASK HIM TO VERIFY THAT WE WERE CLRED TO 3000 FT BUT I NEVER GOT THE WORDS OUT. LESSON: WHEN IN DOUBT; NO MATTER HOW SMALL; DON'T BE AFRAID TO ASK FOR CLARIFICATION!