LTT OVERSHOT ALT INTENDED BY ARTCC DURING DESCENT.

Narrative

WE WERE CLRED FROM FL350 TO FL330 WHICH WAS ACKNOWLEDGED BY COPLT. AFTER CLRNC GIVEN CTLR ADVISED 'MAINTAIN 2-5-0 FOR SEP.' COPLT THOUGHT CENTER MEANT FL250 AND ACKNOWLEDGED; 'ROGER MAINTAIN 250.' CAPT'S (MY) ATTN WAS DIVERTED AT TIME OF LAST BROADCAST AND UPON RETURNING MY ATTN I NOTICED 250 IN ALT SELECT. I QUESTIONED COPLT WHO WAS SURE WE WERE CLRED TO FL250. OUR POLICY IS TO RESOLVE CREW QUESTIONS WITH VERIFICATION FROM ATC. WE WERE GOING TO CALL ATC WHEN ATC CALLED US AND SAID WE WERE CLRED TO FL290. WE WERE GOING THROUGH FL290 AND ADVISED HIM WE WERE CLRED TO FL250. HE SAID; 'ROGER; CLRED TO 250.' PROB COULD HAVE BEEN CORRECTED BY MAKING STANDARD CALLS; SUCH AS 'MAINTAIN AIRSPD 250;' OR GIVE US 'MAINTAIN 250 KTS; MAINTAIN FL330.' OUR PERCEPTION WAS THAT AS WE WERE CLOSE TO FILLMORE VOR WE WOULD GET A QUICK DSCNT TO CROSS TANDY AT 14000'. OUR CREW WILL MUTUALLY UNDERSTAND AND ACKNOWLEDGE ALT CHANGES IN THE FUTURE. ALL CREW CALLBACKS TO ATC WILL CONFIRM EITHER KTS OR ALT WITH THE SPD OR FLT LEVEL.