REPLY TO WRONG CALL SING RESULTS IN A HDG TRACK ALTDEV.

Narrative

THERE WERE 3 ACFT ON FREQ WITH SIMILAR SOUNDING CALL SIGNS. WE ALL HAD BEEN 'STEPPING ON' EACH OTHER'S RADIO XMISSIONS AND ON CTR. WE THOUGHT WE HEARD A 30 DEG R TURN AND CLB TO FL280 (FROM AN INTERMEDIATE STOP AT FL220). WE READ BACK THE CLRNC AND BEGAN TO EXECUTE IT. AT THIS TIME; A COMPANY FLT TOLD CTR THAT SOMEONE HAD TAKEN THEIR CLRNC. CTR THEN GAVE US CLRNC TO FL280. WE THEN ASKED FOR A CONFIRMATION OF HDG HE WANTED US ON AND WERE TOLD TO 'FLY ON COURSE' AT WHICH POINT WE DISCONTINUED THE R TURN AND TURNED BACK L. SEVERAL FACTORS CONTRIBUTED TO THIS INCIDENT: SEVERAL ACFT ON THE SAME FREQ -- ALL OF WHICH WERE GETTING 'STAIR STEPPED' CLRNCS TO THEIR FINAL CRUISE ALT. WE WERE CLOSING TO 10 NM ON THE ACFT IN FRONT OF US AND WERE ANTICIPATING A VECTOR OFF COURSE FOR CONTINUED CLB. SEVERAL RADIO XMISSIONS WERE BLOCKED BY OTHER ACFT BECAUSE A LOT OF INSTRUCTIONS WERE BEING GIVEN AT THE TIME. SUPPLEMENTAL INFO FROM ACN 268345: THE CALL SIGN OF THE OTHER ACFT WAS VERY SIMILAR TO THE CALL SIGN ON OUR LAST LEG. WE WERE BEING 'STAIR STEPPED' BEHIND A PRECEDING ACFT AND COULD SEE ON THE TCASII THAT WE WERE LOSING OUR 10 MI SEPARATION AND WERE THEREFORE ANTICIPATING SOME ACTION VERY SOON BY THE CTLR. IN OTHER WORDS; I WAS COCKED FOR A CLRNC. THIS WAS THE SEVENTH AND LAST LEG OF THE DAY.