FK10 CREW HAD ALTDEV.

Narrative

ALTDEV. I (FO) WAS PF; LEVEL FL310. ATC CALLED ACFT X 'DSND TO FL260; GIVE ME 2000 FPM RATE.' CAPT READ BACK THAT CLRNC AND I CONFIRMED FL260 WITH CAPT VIA POINT AND SHOOT METHOD AT ALT WINDOW. I BEGAN DSCNT. AT FL306; ATC STATED; 'ACFT X MAINTAIN FL310; THAT DSCNT CLRNC WAS FOR ACFT Y NOT YOU.' I QUICKLY RETURNED TO FL310. WE BELIEVE NO LOSS OF SEPARATION RESULTED FROM THE DEV. WE WERE BOTH UNAWARE THAT ACFT Y WAS ON THE SAME FREQ. ATC DID NOT ALERT US TO THIS FACT UNTIL THE DEV OCCURRED. I SUGGEST THAT IT BE STANDARD PROC FOR ATC TO ALERT ACFT WITH SIMILAR CALL SIGNS AND REQUIRE READBACK. IT IS VERY COMMON FOR ATC TO ISSUE CLRNCS TO THE WRONG COMPANY NAME BUT CORRECT FLT NUMBER. THE LAYERS OF CHEESE BROKE DOWN ONCE AGAIN. SUPPLEMENTAL INFO FROM ACN 449731: WE WERE LEVEL AT FL310. I BELIEVE CTR SAID; 'X DSND TO FL260 AND MAINTAIN AT LEAST 2000 FPM IN THE DSCNT.' I CALLED BACK SAYING; 'ACFT X; DSND TO FL260; MAINTAIN 2000 FPM OR MORE IN THE DSCNT.' ATC DID NOT RESPOND. UPON REACHING FL306; CTR CALLED AND SAID THE DSCNT CLRNC WAS FOR ACFT Y. WE TOLD ATC WE WERE AT FL306 AND IMMEDIATELY CLBED BACK TO FL310; WE BELIEVE; WITHOUT INCIDENT. I SHOULD NOT HAVE ACCEPTED THE DSCNT CLRNC WITHOUT A FULL CALL SIGN; IF ATC DID NOT GIVE ONE IN THE DSCNT CLRNC. IF THEY DID; I NEED TO PAY BETTER ATTN TO CALL SIGNS. ALSO; I WOULD HAVE PREFERRED ATC CATCH THE MISTAKEN CLRNC IN MY READBACK.