First, download colobox.pub, and place it in /var/lib/asterisk/keys/ (assuming a Debian installation). Then add and customize the following sections:
[general] register => 8999:YOURPASSWORD@pbx.colobox.com ; The name "iaxcolobox" is required [iaxcolobox] type=user context=from-colobox auth=rsa inkeys=colobox
[from-colobox] exten => 8999,1,Ringing exten => 8999,2,Wait,1 exten => 8999,3,Noop
Replace 8999 with your Colobox PBX extension, and the line 3 command with whatever you want an incoming call to do.
[default] exten => _123.,1,Dial(IAX2/8999:YOURPASSWORD@pbx.colobox.com/${EXTEN:3},60,r) exten => _123.,2,Congestion
Replace 8999 with your Colobox PBX extension, and YOURPASSWORD with your Colobox PBX password. With this setup your can dial any Colobox PBX extension from your home network by prefixing it with "123".
Coming soon, but you really should be using IAX2 with Asterisk instead.