CZEG ASKS B744 FLC IF THEY CAN ACCEPT A HIGHER ALT. THE FLC ACKNOWLEDGE A CLB CLRNC; BUT ARE CHALLENGED LATER BY CZEG. REVIEW OF TAPES SHOW CZEG CTLR MISSED READBACK BY SIMILAR SOUNDING CALL SIGN.

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WHILE FLYING AT FL310; WINNIPEG CTR INQUIRED WHEN WE WOULD BE ABLE TO CLB TO FL330 AND FL350 FOR TFC PURPOSES. WE RESPONDED THAT WE PREFERRED TO STAY AT FL310; BUT COULD CLB TO FL330 IMMEDIATELY AND CLB TO FL350 IN 2 HRS. ABOUT 15-20 MINS LATER; WINNIPEG CTR CLRED US TO CLB TO FL330 FOR TFC. WE ACKNOWLEDGED THE CLRNC AND CLBED TO FL330. ABOUT 10 MINS LATER; WINNIPEG CTR CALLED AND ASKED US TO CONFIRM OUR ALT. WE CONFIRMED THAT WE WERE LEVEL AT FL330. THE CTLR SAID THAT WE WERE NOT CLRED TO FL330 AND THEN RECLRED US TO FL310. IN A DISCUSSION WITH THE CTLR; HE SAID THAT HE HAD CLRED AN ACR Y TO FL330 ON A DIFFERENT FREQ. ADDITIONALLY; WINNIPEG RELAYED TO US THROUGH EDMONTON CTR THAT THEY PULLED THE TAPES AND HEARD US CONFIRMING OUR CLRNC TO CLB TO FL330 AND TOLD US THAT THE CTLR WAS WORKING 2 DIFFERENT FREQS. AT THE TIME OF THE CLRNC; WE HAD A VERY LIGHT WORKLOAD WITHOUT ANY DISTRS. BOTH PLTS WERE WEARING HEADSETS AND WE BOTH ARE CONFIDENT THAT WE WERE CLRED TO FL330 AS FOR ACFT X. THE WINNIPEG CTLR ASSURED US THAT THERE WAS AT NO TIME A LOSS OF SEPARATION OR ANY OTHER TFC CONFLICT. WE BELIEVE THAT THE CONTRIBUTING FACTORS OF THE EVENT WERE WINNIPEG FIRST ASKING IF WE COULD CLB TO FL330 AND THEN CLRING US (OR SOME OTHER ACFT) TO THAT SAME ALT FOR THE IDENTICAL REASON STATED (DUE TO TFC). ADDITIONALLY; BECAUSE THE CTLR WAS WORKING MORE THAN 1 FREQ; HE WAS LESS LIKELY TO CATCH US RESPONDING TO HIS CLRNC.