If you are using regular analog phone lines in the ISP side, those lines will be connected to a modem bank. Rack-mountable or professional modem banks are recommended for reliability. In this case, the modems are typically connected via RS-232 serial interfaces to the Terminal Server. The terminal server can be a stand-alone box with dedicated hardware and software. The PR3000 has also the advantage of being modular and supporting both remote access server and routing functionality in one box.
Some people prefer to build their own server-based terminal server.
The ISP backbone will typically be an Ethernet LAN that connects all the major components in the network. 10BT interfaces are good enough for the WAN equipment, but you may want to use 100BT for the server connections, specially if you have a lot of traffic. The routers, remote access servers and other servers need to support Ethernet connectivity. As an ISP grows, it needs to be present in other geographical areas to be able to provide dialup access using local phone numbers. Besides providing dialup access to home and business users, the ISP can also provide dedicated connectivity to small and medium business.