Internode's Nodephone is a VoIP service. It's not the cheapest VoIP service available, but it works well. It only has two disadvantages for me: no presentation of caller id except to other Nodephone subscribers (and then only by dialling the appropriate code on each call), and no incoming access from the PSTN.
To configure your SPA-3000 using the information on this page, save it to your hard drive and edit it as follows:
<form action="http://your.spa3000.ip.address/admin/asipura.spa" method="post">and replace your.spa3000.ip.address with the IP address or hostname of your SPA-3000.
Note: This configuration may not work for firmware versions other than 3.1.10d. It definitely won't work for versions 2.x -- Sipura added new configuration options in v3.x.
One of the most difficult things to get right is the various gain settings. If the gain is too low, either you or the caller will be difficult to hear. If the gain is too high, you'll have problems with echo on some calls. The values set on my config page suit the three different phones (Doro Congress 305, Ericsson DECT cordless and an Optus phone) I have at home but they may not suit your phones. You may need to experiment to figure out the optimum settings for your phones. Call quality also depends upon line quality - I have a good quality central ADSL splitter and cat 5 cable throughout the house. If you have poor quality cable an cheap in-line splitters you may have trouble getting good quality voice calls even without the SPA-3000. If that's the case, you have no hope of getting it right with the SPA-3000 in place.
To adjust the gain, start with the FXS port input and output gains on the Regional tab. Adjust the gain for the best sound quality on VoIP calls. If you have more than one handset in parallel, test with all handsets: I've found that some are more sensitive than others to the gain settings.
When you've got the VoIP gain settings correct, you can adjust the PSTN gains to improve the quality of calls switched via the PSTN interface. These gain settings are SPA To PSTN Gain and PSTN To SPA Gain on the PSTN Line tab.
Change only one setting at a time and keep notes as you go. When you have a configuration that works, save it so you can reload it if you need to reconfigure your SPA-3000. When you save it, you'll need to edit the form action as above because the URL in the original HTML is relative, not absolute.