Look in the Backup folder on the SD card, and you'll find backup folders for each of the backups. In the latest one, you'll find a file called comm_and_cal.zip which can be unzipped with any utility that handles zip files. Inside that zip file is a file call addressbook.db, which is an sqlite database of the contacts on the device. You can use plenty of database reader programs to read that and look in the proper tables for the information you're looking for.

The format for the tables involved, but is documented several places. If you're diving for one particular number, doing it by hand is probably worth it. If you want to get all of them, it may be easier to have someone dump the database. There are a few python based apps for the N900 that would probably run on any linux based system, given the right packages are installed.