A B727-200 FLC ANSWERS TO A WRONG CALL SIGN AND CREATES AN ALT EXCURSION GOING INTO THE DEN AREA. CREW HAD BEEN DISCUSSING A FUEL FLOW READOUT THAT HAD THEIR PREVIOUS ATTN. FO JUMPED THE GUN AND SET HIS OWN ALT ALERT.

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READ BACK CLRNC TO DSND FROM 13000 FT TO 11000 FT THAT BELONGED TO ANOTHER FLT. QUERIED THE CTLR TO CONFIRM OUR ASSIGNED ALT. DIRECTED TO MAINTAIN 13000 FT. ACFT HAD DSNDED TO 12400 FT BY THE TIME CLARIFICATION WAS RECEIVED. RETURNED TO 13000 FT. NO TFC CONFLICT. SUPPLEMENTAL INFO FROM ACN 399307: WE WERE ON A VECTOR HDG OF 090 DEGS AT 13000 FT MSL. THERE WAS LITTLE TO NO RADIO TFC AND WE HAD JUST MENTIONED FUEL FLOW ON #1 ENG LOOKED NORMAL NOW (FUEL FLOW ON #1 ENG WAS LOW IN CRUISE COMPARED TO #2 AND #3 ENGS). IN MID SENTENCE; APCH GAVE A VERY CURT CLRNC TO 11000 FT AND WE TOOK THAT TO BE OUR CLRNC. FO SET 11000 FT MSL INTO ALT SELECTOR AND STARTED DOWN. ACKNOWLEDGED CLRNC TO 11000 FT. APCH CTL CAME ON RADIO AND CHASTISED ALL ON FREQ TO 'LISTEN UP -- HE WAS EXTREMELY BUSY AND CALL SIGN HE GAVE CLRNC TO WAS NOT CLOSE TO WHO ANSWERED.' HE INFORMED US THAT 13000 FT MSL WAS OUR CLRNC AND THAT WE ACCEPTED ANOTHER ACFT'S CLRNC. IN TALKING TO DEN TRACON SUPVR AFTER ARRIVING; I WAS ADVISED THAT THE CTLR WAS WORKING 2 FREQS AND WAS EXTREMELY BUSY ON OTHER FREQ. NOT REALIZING THE CTLR'S WORKLOAD AND NOT DEMANDING A CLR CALL SIGN ON OUR PART LED TO THIS DEV.