ALT EXCURSION DURING A SINGLE PLT OP IN A B737; FO BEING ADVISED BY ATC OF INCORRECT CLB CLRNC; 100 NM W OF LBB; NM.

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IN CRUISE FLT AT FL350; IT WAS ACTUALLY THE CAPT'S LEG; BUT I WAS IN FULL CTL SINCE THE CAPT HAD TO EXCUSE HIMSELF. SECONDS AFTER HE LEFT THE COCKPIT; CTR CALLED ASSIGNING FL370 AS OUR NEW ALT. I ACKNOWLEDGED THE CALL AND BEGAN A CLB. PASSING THROUGH APPROX FL365; CTR CALLED ASKING WHAT OUR ALT WAS. I RESPONDED ADDING THAT WE HAD BEEN CLRED TO FL370. THE CTLR SAID THAT THERE MUST HAVE BEEN A MISTAKE AND CLRED US BACK TO FL350. I COMPLIED; AND THE ACFT WAS REESTABLISHED IN CRUISE BACK AT FL350 BY THE TIME THE CAPT RETURNED FROM THE TOILET. CONTRIBUTING FACTORS: AT THE TIME CTR MADE THE CLRNC CALL; I WAS STILL FUSSING WITH MY OXYGEN MASK. IT WAS ON AS REQUIRED; BUT I WAS PREOCCUPIED WITH TRYING TO GET IT ADJUSTED SINCE IT WAS INTERFERING WITH MY BOOM EAR PIECE. UNREALIZED AT THE TIME; CTR WAS ALSO WORKING A SIMILAR SOUNDING FLT NUMBER. CTR MAY HAVE GIVEN THE ALT CLRNC USING THE WRONG FLT NUMBER. ON THE OTHER HAND; THOUGH I OBVIOUSLY DID NOT THINK SO AT THE TIME; IT'S ENTIRELY POSSIBLE THAT I RESPONDED TO A CLRNC MEANT FOR THE OTHER FLT NUMBER. IN HINDSIGHT; I SHOULD HAVE QUESTIONED THE WRONG-WAY ALT. ACCORDING TO OUR FLT PLAN; I KNEW WE NEEDED TO CLB TO FL390; BUT WE HADN'T MADE THE REQUEST YET.