CRJ FLT CREW TAKES THE CLRNC INTENDED FOR AN ACFT WITH A SIMILAR SOUNDING CALL SIGN.

Narrative

ON DOWNWIND VECTOR FOR IAD RWY 1L; THE CAPT AND MYSELF EXECUTED A SIMILAR SOUNDING CALL SIGN (SAME COMPANY) CLRNC. THE CLRNC CAME AS A L TURN TO 040 DEGS TO JOIN THE RWY 1L LOC. I TURNED THE ACFT (AUTOPLT) AS THE CAPT READ BACK THE CLRNC TO ATC. AFTER SOME 'BLOCKED' COM; OUR OTHER COMPANY ACFT INFORMED ATC THAT SOMEONE ELSE MAY HAVE TAKEN THEIR CLRNC. AT THIS POINT WE WERE PASSING THROUGH A 160-170 DEG HDG AS ATC TURNED US BACK R TO A 200 DEG HDG; FOLLOWED BY A FURTHER R TURN TO 220 DEGS. ATC THEN ISSUED TFC DSNDING IN FRONT OF US WHICH WE BOTH ACQUIRED VISUALLY AS THE TCASII ISSUED A TA. THE FLT LANDED WITHOUT FURTHER INCIDENT. BOTH THE CAPT AND MYSELF THOUGHT THAT THE CTLR HAD ISSUED THE CLRNC USING OUR CALL SIGN. WE MAY HAVE THOUGHT THIS AS WE WERE FOLLOWING THE OTHER COMPANY TFC INBOUND ON THE ARR AND MAY HAVE GOTTEN USED TO HEARING THAT CALL SIGN. THIS EVENT SHOWS THAT ETERNAL VIGILANCE IS THE KEY TO SAFETY. SUPPLEMENTAL INFO FROM ACN 617666: I READ BACK THE VECTOR AND THE FO INITIATED THE TURN. THE FO ALSO CONFIRMED THE VECTOR WAS FOR US. BOTH MYSELF AND FO FELT THAT THE APCH CTLR MISTAKENLY USED OUR CALL SIGN WHEN GIVING US A TURN TO INTERCEPT. THE TURN WAS A SENSIBLE VECTOR FOR US. THE READBACK BY MYSELF WAS NOT CHALLENGED BY THE APCH CTLR.