RETURNING TO FLT DECK; B757 CAPT IS BRIEFED BY FO HE ACKNOWLEDGED DSCNT CLRNC. ZLC QUESTIONS B757 ADVISING CLRNC FOR ANOTHER ACR. RETURNING FO ALLEGES USE OF CALL SIGN IN READBACK.

Narrative

WE WERE LEVEL AT FL350 ENRTE FROM JFK TO LAS. I LEFT THE FLT DECK TO USE THE LAVATORY. UPON RETURNING; THE FO BRIEFED ME THAT ZLC HAD CLRED US TO DSND AND MAINTAIN FL280. THE ACFT BEGAN AN AUTOPLT DSCNT AS THE FO LEFT THE FLT DECK TO USE THE LAVATORY. AS THE ACFT DSNDED THROUGH FL345; THE ATC CTLR ASKED IF WE WERE LEVEL AT FL350. I REPLIED THAT WE WERE DSNDING TO FL280. THE CTLR ADVISED THAT WE WERE NOT CLRED TO DSND AND XING TFC WAS AT OUR 1 O'CLOCK POS AT FL330. I DISENGAGED THE AUTOPLT AND QUICKLY BROUGHT THE ACFT BACK TO FL350. THE ACFT AT FL330 WAS IN SIGHT AND TCASII INDICATED THAT IT WAS AT 1200 FT BELOW US. THE CTLR ADVISED THAT THE DSCNT CLRNC WAS FOR ANOTHER ACR. THE OTHER ACR FLT CREW INDICATED THAT THEY DID NOT DSND AND THAT THE CLRNC HAD BEEN TAKEN BY SOMEONE ELSE. THE FO CLAIMED THAT HE DID READ BACK THE DSCNT CLRNC USING THE FULL CALL SIGN. SINCE I WAS NOT ON THE FLT DECK WHEN THE CLRNC WAS GIVEN; THE ONLY THING I MIGHT HAVE DONE WAS TO VERIFY THE CLRNC WITH ATC WHEN I ARRIVED BACK ON THE FLT DECK. THIS IS WHAT I WILL DO IN THE FUTURE; EVEN THOUGH IT MAY BE CONSTRUED AS A LACK OF TRUST IN MY FO.