programming procedures for the voicemail portion of the nam is the same as the flash.
I only have a few nams out there and have always set the greetings at the costumers site.
here's a thought,
if you have a rad installed at the costumers site, walk them through on how to set an alt greeting then go in with the remote utility to change the schedules.
what I usually do if a costumer needs small changes is have them call me one phone (cell, fax, or a phone next to a 7310 or 7324) and I walk them through the programming