AsteriskNow is a well known Open Source telephone system. Introduced by Digium, the creator and primary developer of Asterisk, AsteriskNOW includes all the software needed to configure a phone system that offers the advanced features found in expensive proprietary products.
This video simply explains how to install and carry-out basic configuration. AsteriskNow is very adaptable and can be used for many purposes. Also watch the video on Lifehacker on how to hack your home telephone with Asterisk.
AsteriskNOW is licensed under the open source general public license and can be download at the AsteriskNow homepage