Asterisk + X-lite without VoIP provider?

Hi all,

Where I can find the configuration only for the internal communications ? I would like use the Asterisk with X-lite clients. I want to connect three locations of firm without the external VoIP provider, where anyone can call to each.
Someone can help me ? Maybe someone have this configuration ? Maybe any examples ?

(sorry for my english)

I don’t understand the difficulty. Why can’t you use the same configuration, except without the parts relating to the VoIP provider?