B757 DEP FROM CLT ACCEPTS CLRNC FOR SIMILAR CALL SIGN.

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ACR X DEPARTED CLT. XA00 EDT; HORNET DEP WITH SEVERE TSTMS IN AREA. CONTACTED CLT. DEP CTL 120.5; CLBING TO 8000 FT. THEY GAVE US HDG 200 DEGS. THE NEXT CLRNC WE HEARD AND RESPONDED TO; TURN HDG 290 DEGS AND CONTACT ZTL 135.35. WE WERE LOOKING AT RADAR WITH BAD WX AT 12 O'CLOCK POS AND GET OVER TO CTR. CALLED ZTL 'ACR X' LEVEL 8000 FT; WOULD LIKE TO DEV 10 DEGS L FOR WX. ZTL RESPONSE; TURN L 10 DEGS; CLB FL230; AND DIRECT ATL VOR WHEN ABLE. WE READ BACK CLRNC AND REPLIED ACR X. ABOUT 45 SECONDS LATER; WE WERE LOOKING AT OUR RADAR AND SAW TFC AT 12 O'CLOCK DSNDING AT US. WE TOOK EVASIVE ACTION AND STARTED A DSCNT BACK FROM 9100 FT TO 8000 FT; AND RECEIVED A TA ONLY ON OUR TCASII. I CALLED ZTL BACK AND SAID WE ARE DSNDING BACK TO 8000 FT; BECAUSE OF TFC. IS THERE A PROB? AFTER A 45 SECOND WAIT; ZTL SAID MAINTAIN 9000 FT. I TOLD HIM WE READ BACK CLRNC AND ANSWERED ACR X. CHK YOUR TAPES! ZTL REPLIED 'IF I KNEW YOU WERE GOING TO TURN THAT FAR W; I WOULDN'T HAVE CLBED YOU TO FL230. (HE THOUGHT WE WERE FLT Y IN FRONT OF US); AND SAID THERE SHOULD BE NO PROB SINCE SEPARATION WAS 3 MI AND 1400 FT ALT DIFFERENCE. WHEN HANDING US OFF TO ANOTHER ZTL FREQ; HE SAID CAUTION THERE IS AN ACR Y ON FREQ. THAT WAS THE FIRST TIME WE HEARD FROM DEP CTL OR ZTL OF A LIKEWISE CALL SIGN! THE NEXT CTR FREQ TOLD US TO CALL CHARLOTTE DEP CTL WHEN WE GOT TO OUR DEST OF PHX. WHEN I ARRIVED AT PHX; I TALKED TO CLT DEP CTL SUPVR. HE TOLD ME YOUR COMPANY HAS HAD ONGOING PROBS WITH SIMILAR CALL SIGNS; AND THAT NIGHT WITHIN 8 MINS THERE WAS ACR X; Y; AND Z ON FREQ. HE TOLD ME THAT 'YOU TOOK ACR Y'S CLRNC; 290 DEGS AND ZTL 135.35 AND NEVER READ BACK YOUR CALL SIGN OF ACR X;' SO HE WAS GOING TO WRITE-UP A PLT ALTDEV FOR TAKING THE WRONG CLRNC AND NOT READING YOUR FLT NUMBER BACK. HE SAID ZTL HAD TO GO THROUGH THEIR TAPES TO SEE WHAT HAPPENED; AND WOULD GET BACK LATER. HE ALSO SAID 'NO ALT SEPARATION WAS BROKEN OF LESS THAN 1 MI OR LESS THAN 1000 FT!' WE RESPONDED TO A SIMILAR CALL SIGN; BUT YOU WOULD THINK AFTER THAT AMOUNT OF TIME; AT LEAST 2 MINS BEFORE WE STARTED A CLB; SOMEONE WOULD HAVE TOLD US ACR X WAS ON THE WRONG FREQ AFTER CHKING IN TWICE TO CTR WITH READBACK ACR X! (3 EVENTS) 1) SIMILAR CALL SIGNS; 2) BAD WX; 3) 2 DIFFERENT FREQ.