A B727 CAPT RPTED THAT HE HAD A SIMILAR SOUNDING CALL SIGN TO THAT OF A CARGO CARRIER. THE CARGO CAPT BECAME CONFUSED AND MISSED A LEVELOFF CALL FROM ATC.

Narrative

A CONFLICT/LOSS OF SEPARATION OCCURRED BTWN 2 ACFT; 1 PAX; 1 CARGO. PAX ACFT IN LEVEL FLT; FL230; AND CARGO ACFT IN INITIAL CLBOUT. DUE TO ANOTHER (MY) ACFT WITH SIMILAR CALL SIGN; THE CARGO ACFT MISSED A LEVELOFF CALL FROM ZTL. I RESPONDED TO THE CALL DUE TO SIMILAR FLT NUMBERS (FLT ABC AND FLT ZBC). THERE WAS A HVY VOLUME OF TFC ON THE FREQ. BECAUSE OF THIS; I WAS UNABLE TO VERIFY THE ALT ASSIGNMENT. I WAS UNAWARE OF THE OTHER ACFT (FLT ABC) ON THE FREQ UNTIL THE CTLR ASKED HIM TO VERIFY HIS ALT. FLT ABC RESPONDED FL215. THE CTLR THEN ASKED FLT ABC TO RPT OUT OF FL213 AND HE RESPONDED WE ARE PASSING FL217. ATC AGAIN SAID RPT LEAVING FL213. FLT ABC RESPONDED 'WE ARE OUT OF FL220.' THE PAX FLT WAS LEVEL AT FL230; OPPOSITE DIRECTION TO THE CLBING CARGO FLT. THIS RESULTED IN A 'TCASII RA' FOR THE PAX FLT. THE 2 ACFT PASSED WITHIN 500 FT AS RPTED BY THE PAX ACFT. WE WERE CLBING TO ASSIGNED FL230 WHEN THE CTLR ASSIGNED FL210. WE RESPONDED; READ BACK CALL SIGN AND NEW ALT; AND SET ALT IN THE ALT WINDOW. WE BECAME AWARE OF THE POTENTIAL MISHAP WHEN ATC BEGAN ISSUING FRANTIC ALT RPTS TO FLT #ABC. WE CONTINUED OUR CLB TO FL230.