SW3 FLT CREW WITH ZFW AT APPROX 140000 FT EXPERIENCED CONFLICT BECAUSE OF UNAUTH DSCNT WHEN CLRNC WAS ISSUED TO SIMILAR SOUNDING ACFT IDENT.

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WHILE PROCEEDING DIRECT SLUGG INTXN ON THE SLUGG 5 ARR INTO DTO AT FL210; WE WERE GIVEN A PLT'S DISCRETION CLRNC BY ZFW TO 15000 FT AND TO EXPECT CLRNC TO CROSS SLUGG AT 4000 FT. FTW TOLD US TO BEGIN DSCNT AT THE SAME TIME WE CALCULATED WE SHOULD BEGIN DSCNT TO MAKE THE EXPECTED XING RESTR. PRIOR TO REACHING 15000 FT; THE CAPT (PF) AND MYSELF PERFORMING COPLT DUTIES (PNF) BOTH COPIED AND I READ BACK A CLRNC FROM FTW FOR US TO DSND TO 6000 FT. I THEN ASKED ATC TO RECONFIRM THE DSCNT FOR ACFT X SINCE WE WERE GIVEN TFC OFF TO OUR R AT 14000 FT. WE BOTH UNDERSTOOD ATC TO RECONFIRM OUR CLRNC TO 6000 FT ASSIGNMENT. AT THAT POINT; I WAS THINKING THAT MAYBE THE OTHER TFC; WHICH I THINK WAS A KING AIR; WAS VFR AND EITHER CLBING OR DSNDING THROUGH 14000 FT AND THAT THE TFC CALLOUT WAS ADVISORY ONLY. WHILE DSNDING AT 2000 FPM TO COMPLY WITH THE EXPECTED XING RESTR AND PASSING THROUGH 14300 FT; ATC TOLD US TO MAINTAIN 15000 FT WHICH WAS IMPOSSIBLE AT THAT POINT. WE WERE ALREADY THROUGH 14000 FT FOR 6000 FT. WE TOLD ATC OF OUR SIT AND QUIZZED HIM AGAIN ABOUT OUR PREVIOUSLY UNDERSTOOD CLRNC TO 6000 FT. THE ATC CTLR DENIED GIVING US 6000 FT AND SAID THE 6000 FT ASSIGNMENT WAS FOR ANOTHER ACFT WHOSE TAIL NUMBER I DID NOT LEARN OTHER THAN IT ENDED WITH A DISCRETE SUFFIX. NEITHER OF US HEARD ANY CLRNC FOR SUCH A TAIL NUMBER. WE DID ACQUIRE THE OTHER ACFT VISUALLY WHEN PASSING THROUGH ABOUT 14000 FT AND SEPARATION DID NOT SEEM VERY CLOSE. I'M QUITE CERTAIN THAT FREQ BLOCKAGE AT A VERY INOPPORTUNE TIME WAS THE ROOT CAUSE OF THIS ASSUMED ALTDEV. IN ADDITION; A SIMILAR SOUNDING CALL SIGN CAUTION ALERT WAS NOT PROVIDED TO US BY ATC WHICH IS NORMALLY PROVIDED; WAS LIKELY A CONTRIBUTING FACTOR. THE CAPT AND I CONSIDER OURSELVES PROFESSIONAL PLTS AND WE REGRET THE MISUNDERSTANDING AND LOSS OF SEPARATION. WE ARE NOT IN THE HABIT OF DISREGARDING CLRNCS. FREQ CONGESTION; BLOCKED XMISSIONS AND SIMILAR CALL SIGNS JUST DON'T MIX IN THE ATC ENVIRONMENT TODAY AND A REMEDY NEEDS TO BE FOUND SOON.