An update and hopefully the end of this thread. I was back out on site today and after talking to my friend who had asked me for help, I used the "Station Pair" function to make extension 209 the secondary station to 207. As such, if someone calls, or is transferred by the Auto Attendant or by another extension to 207 (the original and main phone for the restaurant), both 207 and 209 will ring, and if there's no answer then the caller can leave a message on 207. About the only thing this doesn't easily allow is the notification of and checking of new messages from 209, but they're fine with that -- 209 is at a desk not always staffed, 207 is the main phone used.
The bonus of this is that I didn't have to attempt to tweak the settings for the Auto Attendant, which obviously was set up by the original installer, and is not something I've ever worked on before.
For kicks, I did play around with setting up a button to check the 505 group voicemail box, just to see how to do it. The function was, as Gerrya mentioned above, "VG505".
Thanks to everyone who posted and messaged me with help on this -- you all are very friendly and helpful.
