A B737 FLC IS UNSURE OVER THEIR LAST CLRNC WHEN THE SAME CLRNC IS GIVEN TO ANOTHER COMPANY FLT. WHILE QUERYING ATC THEY START A RETURN TO THEIR PREVIOUS ALT WHILE 150 MI E OF ORL; FL.

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IN RADIO CONTACT WITH ZMA. STRAIGHT AND LEVEL AT 23000 FT MSL ON AN ASSIGNED HEADING 060 DEGS. A CLRNC WAS HEARD TO CLB TO 29000 FT. UNSURE OF THE CALL SIGN; FO QUERIES ZMA. ZMA RESPONDS THAT THE CLRNC WAS FOR UNITED STATES BDHA; AND ALSO MUST TURN TO 030 DEGS. THE TURN AND CLB ARE INITIATED. NEXT; THERE ARE SOME SEEMINGLY BLOCKED RADIO XMISSIONS; AND ZMA MAKES A XMISSION TO THE EFFECT; 'ACR BDFA THIS CLRNC IS FOR YOU; CLB TO 29000 FT AND TURN TO 030 DEGS. I HAVE TRIED TO CALL YOU 4 TIMES.' DUE TO THE SIMILAR CALL SIGNS AND EXACT CLRNC TO THE OTHER ACFT; FLT BDHA DISENGAGES AUTOPLT; DISCONNECTS CLB AND TURN; AND RETURNS TO LAST ASSIGNED HEADING 060 DEGS AND ALT 23000 FT; NOW BEING UNSURE IF WE HAVE ACKNOWLEDGED THE APPROPRIATE CLRNC. AFTER A FEW ATTEMPTS TO CLARIFY; FLT BDHA IS ABLE TO DETERMINE THEY HAD RECEIVED THE CLRNC TO TURN AND CLB. AT THIS TIME; SURE OF THE CLRNC INSTRUCTIONS ARE COMPLIED WITH. CONTRIBUTING FACTORS: SIMILAR CALL SIGNS: FLTS BDHA; BDFA. BOTH ACFT ON SIMILAR ROUTING; SAME FREQ; RECEIVE SAME CLRNC FROM ZMA WITHIN A VERY SHORT PERIOD OF TIME; IN FACT ALMOST SIMULTANEOUSLY.