How to set up a call server from pc to pc?

Good day to all,
How to set up a call server from pc to pc? I have a newly build company that deals with simple office works. I was just thinking to set up a call server from pc to pc by networking methods, but I am not so sure on how to get this working. We have two buildings which is 25 meters apart, so it would be better if we just make a call from server to shared computers. We want the conversation to be encrypted end to end for security purposes. How can we make a call without an internet server like Skype or yahoo. Can you help me out of this?
Thanks.
