If you do not need to connect to the PSTN, Asterisk requires no hardware other than a server with a Network Interface Card.

However, if you are going to be providing music on hold^[34] or conferencing and you have no physical timing source , you will need the ztdummy Linux kernel module. ztdummy is a clocking mechanism designed to provide a timing source to a system where no hardware timing source exists. Think of it as a kind of metronome to allow the system to mix multiple audio streams in a properly synchronized manner.