Grandstream UCM6100 series – A disruptively priced new system from Grandstream.
This asterisk based PBX offers: inexpensive, low power consumption ∴ long UPS runtime. It the UCM6104 has 4 analog ports which can be used for internal fax or PSTN analog ports (for example your current Comcast lines) which might ease porting & integration: simultaneous PSTN or IP participants.
GrandStream UCM6104 IP PBX appliance ≈ $300 amazon