Asterisk PBX

If you want to create your own internal PBX then your best bet is to use Asterisk, available here:

http://www.asterisk.org/

Here's a comment on Slashdot with an idea how to merge VOIP and POTS into an Asterisk system in order to solve the 911 and power outage problems inherant in VOIP:

http://slashdot.org (abbreviated URL for aesthetics)