The Linksys/Sipura SPA-3000 is a phone adapter with one FXS and one FXO port (along with an RJ-45 ethernet port as well). Any standard analog telephone can be plugged into the FXS port and any standard POTS/PSTN line can be plugged into the FXO port.
As far as Asterisk is concerned, the FXS port is simply a SIP extension (in my case, 202). The FXO port is a little trickier because you have to configure it in Asterisk as a trunk. The extension part works just like any other extension in Asterisk - it can be part of a queue, ring group, voicemail, etc. The trunk also works like any other trunk, however most people would use it as a backup or for reliable 911 service (that's what I use it for also).
If you could make it part of Pluto for easy (or even auto) configuration, that would be excellent. But that doesn't seem to be the main problem. When I originally was investigating this problem, I set up a softphone on my laptop (which is the same to Asterisk as the Sipura) and it did not ring either. When I looked into the Asterisk configuration files (namely extension_pluto_dial.conf), I saw that any phone that I added which was not part of a Media Director, was not included in the list to ring when there is an incoming call. This situation is undesirable and I have not found any configuration options to correct it through Pluto Admin. I think this may be a bug, but I did not want to report it as such until I found out if I was simply missing some flag/config option somewhere.
Sorry to be long-winded, but I appreciate your quick feedback and I will provide any more information you would like about the SPA-3000, but I think the problem goes beyond the phone adapter to just any SIP phone that I add is not added to the ring list in Asterisk config. Thanks for your time.