Newbie- intercom

Hi all,
Im a newbie in asterisk. I want to setup an intercom in my college. I have four phones in different departments. The departments are connected using network cables(RJ45). and also telephone cables.How can I setup intercom using asterisk in my college. Please help me out.Is there ant additional hardware needed.
Thanks in advance

What kind of phones will you be using ? If it is phone to phone you can just dial an extension and send a sip header to the phone telling it to auto pick up. It is not hard to do.

Yes, I would like to call from one phone(analogue) to another phone in my college. How can I do that with asterisk.Help me.