I "may" be wrong, but the only back-up you can do of a "Nortel Norstar PLUS Modular ICS telephone system" is... jack.
You CAN back-up the voicemail (depending on flash/nam), but your "Nortel Norstar PLUS Modular ICS telephone system" doesn't have any back-up capability.
Plus, any tech worth his salt would have your system reprogrammed and running within hour of a default anyways.
However, you might want to double check those assertions in the "Nortel: Norstar systems" forum.
Have a nice day.