Need help to configure the freepbx

I am new to asterisk and freepbx. I installed everything on CentOs 5.3: Asterisk 1.6.2 and FreePBX 2.6RC2.Asterisk is running, freepbx GUI is also working.

I defined four zap ports for four FXS ports on TDM card as “from-internal”.
In FreePBX, I defined four extensions.

However, when I tried call from one extension to another, I can only get busy tone.

Where are the best place for a newbie to start with this? I am looking for a step by step guide that I can follow to get this simply phone system with four extension and three POSt line and IVR up and running. But so far, I can see a lot of document, but not in the basic sequence for people not familar with this.

Thanks for your help!