ARTCC CLRED FLT XYY TO DSND INSTEAD OF FLT XXY; SIMILAR FLT NUMBERS.

Narrative

ZMP ASKED ACR XYY TO DSND FROM FL330 TO FL290 FOR XING TFC. FO READ BACK CLRNC TO FL290 AND RPTED LEAVING FL330 USING OUR CORRECT CALL SIGN. ATC ACKNOWLEDGED OUR READBACK. NEXT WE HEARD ACR XXY CHKING IN WITH ATC. AT THAT TIME ATC CALLED US BACK TO CONFIRM THAT WE WERE STILL AT FL330. REALIZING WHAT HAD PROBABLY HAPPENED THE CAPT (FLYING) STARTED AN IMMEDIATE CLB OUT OF OUR PRESENT ALT OF FL323 BACK UP TO FL330. ATC THEN ADVISED US OF A POSSIBLE PLTDEV AND SAID TO CALL ZMP ADMINISTRATIVE OFFICES AFTER XA00 AM CDT ON AUG/TUE/96. EXTENUATING CIRCUMSTANCES INCLUDE: 1) ATC ISSUED DSCNT CLRNC BEFORE ACR XXY HAD EVEN CHKED IN ON FREQ. 2) WAS NEVER ADVISED (PRIOR TO THE ALLEGED DEV) THAT THERE WERE SIMILAR SOUNDING CALL SIGNS ON FREQ. 3) ATC REFERRED TO OUR FLTS AS ACR XYY AND ACR XXY INSTEAD OF ACR XXY AND ACR XYY; RESPECTIVELY. 4) ATC ACCEPTED OUR READBACK AND THE FACT THAT WE WERE LEAVING FL330. 5) WARNINGS OF SIMILAR SOUNDING CALL SIGNS FOR THESE 2 FLTS IS NORMALLY PART OF THE RELEASE BUT FOR SOME REASON THIS WARNING DID NOT APPEAR ON ANY OF OUR RELEASES. IN CLOSING; AFTER REVIEWING ATC TAPES THE FOLLOWING DAY; ZMP ACCEPTED FULL RESPONSIBILITY FOR THE SIT AND ACCEPTED AS TRANSCRIBED ABOVE AS FACT.