After lots of trying and giving up, I've finally configured an asterisk server on my Ubuntu server. I got it to accept connections from my SIP client, and to connect to XS4ALL's "Bellen" SIP service.
First, the easy bit: installing asterisk. You can do this with the following command: apt-get install asterisk (note: as Joe Terranova points out, you should pay attention to the security side of this. Installing on a machine inside your LAN isn't too much of a risk, but if you install it on a machine with a public IP address, make sure you have all security fixes installed, and set up iptables.)
Then you have to configure the beast. This can look hard, because there are almost 80 config files in /etc/asterisk. Furtunately, you can ignore most of them for now.
First, add a SIP account to sip.conf. An entry for a SIP account look like this:
[connection_nickname] type=peer nat=yes ; Or "no", if your client has a public IP username=the_username secret=password123 canreinvite=no ; Ekiga needs this context=default
Once you've configured this, restart asterisk, a