If you are going to connect Asterisk to any legacy telecommunications equipment, you will need the correct hardware . The hardware you require will be determined by what it is you want to achieve.
Asterisk allows you to seamlessly bridge circuit-switched telecommunications networks with packet-switched data networks. Because of Asterisk’s open architecture (and open source code), it is ultimately possible to connect any standards-compliant interface hardware. The selection of open source telephony interface boards is currently limited, but as interest in Asterisk grows, that will rapidly change. At the moment, one of the most popular and cost-effective ways to connect to the PSTN is to use the interface cards that evolved from the work of the Zapata Telephony Project (http://www.zapatatelephony.org).
Unless you need a lot of channels (or a have lot of money to spend each month on telecommunications facilities), chances are that your PSTN interface will consist of one or more analog circuits, each of which will require a Foreign eXchange Office (FXO) port.
Digium, the company that sponsors Asterisk development, produces the most popular analog interface card for Asterisk , known as the TDM400P. The TDM400P is a four-port base card that allows for the insertion of up to four daughter cards, which deliver either FXO or Foreign eXchange Station (FXS) ports. The TDM400P can be purchased with these cards preinstalled, ...