you could try here:
PBX-Guide: Find the IP-PBX appliance that's right for you I would get a UPS that would keep the system running for at least 4 hours, the vendor you choose should be able to help size it for you. They can also help you with the type of application and conections required to manage it. Some of the newer systems use web interfaces.
If you want to explore open source, check out:
Asterisk :: The Open Source Telephony Platform | Setting Up Asterisk, With John Todd — Alec Saunders .LOG