CL60 FLC EVIDENTLY ACKNOWLEDGED AN ATL CLB CLRNC FOR A COMPANY ACFT WHICH IS RECOGNIZED ONLY AFTER FREQ CHANGE TO ZTL.

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PF WAS LEVELING AT 3000 FT HDG 090 DEGS. UPON CHK-IN WITH DEP; ATL DIRECTED A CLB TO 4000 FT; SAME HDG. PNF SET ALT ALERTER TO 4000 FT AND READ BACK CLRNC. THEN THE CTLR DIRECTED A CLB TO 14000 FT AND FREQ CHANGE TO ZTL ON 134.5. PNF READ BACK THE CLRNC TO CLB TO 14000 FT AND CHANGE TO 134.5 FOLLOWED BY CALL SIGN COMPANY X. THEN THE PNF CHANGED THE ALT ALERTER TO 14000 FT. NO FURTHER INSTRUCTIONS WERE RECEIVED FROM DEP CTL. UPON MONITORING CTR FREQ; WE HEARD A COMPANY ACFT; COMPANY 2; CHKING IN ON CTR FREQ. THEY; TOO; WERE CLBING TO 14000 FT. IT WAS UNUSUAL TO HEAR ONE OF OUR OTHER COMPANY ACFT SINCE OUR FLEET ONLY HAS 8 ACFT. AFTER COMPANY Z CHKED IN; PNF COMPANY 1 CHKED IN WITH CTR; ADVISING THAT WE WERE HDG 090 DEGS; CLBING TO 14000 FT. CTR SAID WE WERE NOT TO BE ON THIS FREQ; RETURN TO PREVIOUS CTLR. PNF IMMEDIATELY WENT BACK TO DEP ON 119.3. CTLR SAID TO LEVEL AT 10000 FT; WHICH PF DID. ACFT CONTINUED TO BE VECTORED AT 10000 FT WITH THIS CTLR. HDOF WAS MADE TO A SUBSEQUENT APCH CTLR FOR FURTHER VECTORS. ONCE S OF THE ATLANTA COMPLEX; APCH CLBED US TO 14000 FT AND HANDED US OFF TO CTR ON 134.5. CHKING IN WITH ZTL; WE WERE GREETED WITH WELCOME BACK. WONDERING WHERE THINGS WENT WRONG ON THE CLRNC; I CONTACTED COMPANY 2 ON OUR COMPANY FREQ. THEY INFORMED ME THEY HAD JUST DEPARTED FALCON ARPT IN ATLANTA AND HAD BEEN TALKING TO ATL APCH ON 119.8; DIFFERENT FROM OUR 119.3. THEY RECEIVED THEIR CLRNC TO 14000 FT AND HDOF TO CTR ON 134.5 ON THAT FREQ. I AM STILL CONFUSED AS TO WHAT REALLY ALL HAPPENED BEHIND THE SCENES OF ATC. FORTUNATELY; IT WAS VMC AND WE WERE TCASII EQUIPPED. VOLUME ALSO SEEMED TO BE VERY LOW. I CAN ONLY SPECULATE; THE DEP CTLR WAS WORKING 2 FREQS SIMULTANEOUSLY. I NEVER HEARD ANY DIRECTIONS TO COMPANY 2 UNTIL INITIALLY MONITORING 134.5 (ZTL). MY INITIAL READBACK TO DEP; ACKNOWLEDGING A CLB TO 14000 FT AND FREQ CHANGE TO 134.5; WAS NOT CHALLENGED. THERE WAS AMPLE TIME BTWN ACKNOWLEDGEMENT AND ACTUAL FREQ CHANGE. IF THE CTLR HAD BEEN WORKING 2 FREQS SIMULTANEOUSLY; THERE COULD HAVE BEEN CONFUSION CAUSED IF BOTH COMPANY 1 AND 2 ANSWERED SIMULTANEOUSLY. MAYBE MY ACKNOWLEDGEMENT WAS NEVER HEARD. I WAS NEVER WARNED THAT ANOTHER ACFT WITH SIMILAR CALL SIGN WAS BEING WORKED BY THE SAME CTLR. THIS ENTIRE EVENT COULD PROBABLY NOT EVEN HAPPEN THIS WAY AGAIN. THE ODDS ARE TOO GREAT. IT ONLY POINTS OUT THAT TRUE DILIGENCE ON BEHALF OF BOTH PLTS AND CTLRS IS REQUIRED TO AVOID PROBS ON CONFLICTS. WE NEED TO RECOGNIZE WHEN POTENTIAL PROBS CAN OCCUR AND TAKE NECESSARY STEPS BEFORE PROBS CAN MATERIALIZE. THE PERSON WITH THE MOST INFO HAS THE ADVANTAGE IN THIS SIT. THIS WHOLE EVENT SERVES TO POINT OUT THAT WE SHOULD NEVER LET OUR GUARDS DOWN. WE HAVE THE BEST SYS IN THE WORLD. WE NEED TO WORK EVEN HARDER TO KEEP IT THIS WAY. VOLUME CAN ONLY GET GREATER AND EXPERIENCE WILL GET EVEN LOWER.