ALTDEV ALT EXCURSION FROM ASSIGNED AS AIRLINE X LEAVES ASSIGNED ALT IN ANTICIPATION OF HIGHER WHEN A SIMILAR SOUNDING ACFT NUMBER AIRLINE GETS THAT CLRNC.

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AFTER SEVERAL STEP CLBS; ATC GAVE US A FREQ CHANGE AND SAID 'EXPECT FURTHER CLB WITH NEXT CTLR.' I CHKED IN ON THE NEW FREQ AND THE CTLR IMMEDIATELY CAME BACK WITH A CLB TO FL290 FOR OTHER AIRLINE Y ABDH AND DID NOT ACKNOWLEDGE OUR CALL AT ALL. THE CAPT EXPECTING TO HEAR THE CLB AND OUR CALL SIGN BEING AIRLINE XHDH STARTED A CLB. NO PWR WAS ADDED SO I DIDN'T CATCH IT TILL WE WERE AT FL233 AT WHICH TIME I TOLD THE CAPT TO GO BACK TO FL230 THAT THE CALL WAS FOR SOMEONE ELSE. WE TOPPED OUT ABOUT FL236 BEFORE THE PLANE CAME BACK DOWN. NOTHING WAS SAID BY ATC AND NO CONFLICT WAS NOTICED. IF ATC HAD JUST SAID AIRLINE XHDH STANDBY BEFORE GIVING A SIMILAR CALL SIGN A CLB CLRNC RIGHT AFTER I CHKED IN; THE CAPT PROBABLY WOULD HAVE CAUGHT IT. WE HAVE ALL MISHEARD AND I HAD NOT READ BACK ANYTHING TO ATC WAITING FOR AIRLINE Y TO ACKNOWLEDGE. I HAVE HAD OTHER INSTANCES WHERE ATC DOES NOT ACKNOWLEDGE YOUR CALL AND IMMEDIATELY TALKS TO SOME OTHER ACFT AND IT IS ALWAYS CONFUSING BECAUSE YOU EXPECT THEM TO ANSWER YOU. JUST A QUICK '(CALL SIGN) STAND BY' WOULD HELP. OF COURSE ON OUR PART WE SHOULD TRY AND MAKE SURE WE HEAR RIGHT. I AM SURE THE CAPT THOUGHT HE WAS SURE HE HEARD OUR CALL SIGN AND WAS EXPECTING THAT EXACT CLRNC AND WAS TOLD BY THE PREVIOUS CTLR TO EXPECT A CLB FROM THE NEXT CTLR. LUCKILY I DIDN'T MISHEAR OR WE WOULD HAVE BEEN ON OUR WAY TO FL290.