I have a nokia 5310b

i have a nokia 5310b xpressmusic phone which is locked, i can't remember what the unlock code is and it won't let me access anything to be able to master reset it! the unlock codes for these phones are 10 characters. i tried inserting a different SIM and entering the cell number used with that phone last, only to get a "error" or "enter unlock code" message. their wasn't anything my provider could do and only suggested i send it to the manufacturer but that didn't guarantee any result either! any ideas on how to fix this problem would be much appreciated!