First you'll need a PSTN (Public Standard telephone network) supplier, and one or more routers (depends on the traffic) to handle the incoming and outcoming calls, e.g. a Cisco 1760 router. You can connect up to 4 ports to each card and 4 cards for each router, this are 16 lines. The ports will be assigned to an IP address. When a customer dials the number of your IP address it will be routed to one of the FXS cards (Foreign Exchange Cards) of the Cisco router.