My buddy Darrell explained to me how you really get a poor man’s IP PBX are coming from. Wow you need three boxes, the IP PBX itself, then a box which converts existing analog lines to IP and then a box that goes from the IP PBX back to the analog PSTN.
In order to make it all work with IP Phones, you need Power on Ethernet or POE switch. Now a 48 port switch like the Netgear GS748TP is only $1232 from VOIP Supply and is pretty amazing. 48 ports. Gigabit Ethernet. POE on every port. While more expensive than non-POE, it means that every port you have can be powered, so given a switch lasts a million years, might as well get one. The 24 port version is half the price at about $600, so very nominal. Compared with $!50/hour to configure ports and figure out what is POE and what is not.