I’ve had a spa3000 for a long time and then it blew up. That meant I had to replace it with a spa3102, but without being able to refer to the configuration I had on the spa3000. It wasn’t too bad, although the software on these devices is truly terrible (even with a modern firmware)….
Andrew is playing with Engin and Asterisk. There seem to be plenty of software SIP phones packaged for Debian, so I recommend starting with one of those — the debugging is a lot easier. Also, Nerd Vittles has had some excellent howtos recently.