Open Source Asterisk has already taken IP telephony by storm that made many vendors offering solutions based on the software. This free software also brought down the cost of installing of an IP-PBX. You don't need a fully functional PC for a fully functional IP-PBX, you could use an embedded Asterisk appliance called WARP created by PIKA Technologies.
PIKA WARP Appliance is a development platform which enables reseller and VARs to configure Asterisk 1.4.x to their liking. Mor details on the link below.