I have an old server lying around and I am thinking of using it as an internal telephone server. Its main use would be to set up a 1 to 1 telephone system that could be used internally to connect to different users in different offices.
I have looked at software like Asterisk, but it seems that this is mainly for external telephone systems.
Is this possible to do with Asterisk (or other software) or is this not possible without involving a VOIP company?