B757 FLC TOOK CLRNC FOR ANOTHER ACFT AND STARTED AN ENRTE ALT CHANGE BEFORE BEING ALERTED BY ARTCC CTLR THAT THE CLRNC WAS FOR ANOTHER ACFT.

Narrative

WE WERE CRUISING AT FL280; HAD EXPERIENCED A ROUGH RIDE AND HAD TRIED FL260 AND FL240. WHEN THE RIDE DETERIORATED AT FL240; WE REQUESTED A SMOOTHER ALT AND CTR CLRED US TO CLB BACK TO FL280; OUR FILED ALT. AFTER LEVELING OFF AT FL280; WE COPIED AND I READ BACK A CLB CLRNC TO FL310. CTR RPTED BETTER RIDES AT FL350. AS WE LEFT FL280 CLBING; I ASKED WHAT KIND OF RIDE WE COULD EXPECT AT FL310. CTR RESPONDED THAT THE CLRNC TO FL310 WAS FOR ANOTHER ACFT; BUT THAT FL310 WAS AVAILABLE IF WE WANTED IT. SINCE HE RPTED A ROUGHER RIDE AT FL310; WE STOPPED OUR CLB AT FL290 AND RETURNED TO FL280. CTR RPTED NO LOSS OF SEPARATION. ALTHOUGH WE WERE FILED FOR FL280 ON THIS SEGMENT OF THE FLT PLAN; WE WERE PLANNED TO STEP-CLB TO FL390 AT RZC. BECAUSE OF THE BUMPY RIDES AND THE ALT 'SHOPPING' WE HAD DONE IN SEARCH OF SMOOTH AIR ALREADY; WHEN CTR SEEMINGLY OFFERED FL310 WITH THE PROMISE OF A SMOOTHER RIDE AT FL350; I DIDN'T QUESTION THE CLRNC. I THOUGHT HE WAS JUST BEING HELPFUL OFFERING A HIGHER ALT BEFORE RZC AND JUST READ BACK THE CLRNC. ANOTHER POSSIBLE FACTOR WAS SIMILAR CALLSIGNS. WE NEVER DID FIND OUT WHO THE FL310 CLRNC WAS FOR; BUT EARLIER THERE HAD BEEN AN ACFT XXX AND AN ACFT YYY ON OUR FREQ (WE WERE ACFT ZZZ). WE THOUGHT WE HAD LEFT THEM BEHIND.