B737 CREW HAD ALT EXCURSION.

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SCHEDULED FLT ABCD SEA-OAK. CAPT WAS PF; I WAS PNF HANDLING RADIOS. I AM IN MY FIRST YR WITH MY ACR AND IT WAS MY FIRST TRIP TO/FROM SEA. DURING CLBOUT; OUT OF APPROX 12000 FT; I INFORMED CAPT I WOULD BE OFF COM #1 TO CALL BACK TIMES TO SEA. HE ACKNOWLEDGED (THIS IS SOP). WHILE I WAS ON COMPANY FREQ COM #2; I WAS CONTACTED BY ANOTHER COMPANY ACFT. THEY REQUESTED THAT I PASS ON A CUSTOMER SVC REQUEST TO SEA OPS. THE MESSAGE WAS QUITE EXTENSIVE AND DETAILED; AND THERE WAS ALSO SOME CONFUSION REGARDING DETAILS. IT TOOK SOME TIME AND SEVERAL RELAY CALLS TO COMPLETE. AT LEAST ONCE DURING THIS TIME; I BRIEFED THE CAPT ON WHAT I WAS DOING; AND VERIFIED THAT HE WANTED ME TO CONTINUE OFFERING ASSISTANCE. HE DID. WHEN I RETURNED TO COM #1 AND NORMAL DUTIES; IT HAD BEEN 10-15 MINS AND WE WERE LEVELING AT FL330. I HAD LOST SOME SITUATIONAL AWARENESS AND HE BRIEFED ME ON OUR POS AND LAST CLRNC. HE LEFT COM #1 TO MAKE PAX PA (SOP) AND TURN OFF SEAT BELT SIGN. WHILE HE WAS GONE; ATC QUESTIONED OUR ALT. IN FACT; I HAD DIFFICULTY UNDERSTANDING WHAT THE CTLR WAS SAYING; AND ON THE THIRD ATTEMPT HE STATED 'MODE C SHOWS YOU AT FL336.' BOTH ALTIMETERS SHOWED FL330; HOWEVER; I THEN NOTICED THAT NO ALTIMETER HAD BEEN RESET TO 29.92 AT FL180. WHEN SET STANDARD; WE WERE AT FL336. AT THIS TIME; THE CAPT CAME BACK AND WE DSNDED TO FL330. NOTHING MORE WAS SAID BY ATC; NO OTHER ACFT WERE INVOLVED OR ISSUED DEVS; NOR WERE WE GIVEN ANY DEVS. NO ACFT ON TCASII. I THINK THE CAPT HAD BEEN DISTR LISTENING IN ON COM #2 AND FORGOT TO SET STANDARD ALTIMETER AT FL180. I WAS OUT OF THE LOOP AND DISTR WHEN WE PASSED FL180. I'VE NOW ADDED TO MY LEVELOFF MENTAL CHKLIST TO VERIFY ALTIMETER SETTING AS WELL AS ALT.