Enable activation/deactivation of peripheral devices and various call-handling scenarios
The FXS models support I/O ports. The two output ports enable activation/deactivation of peripheral devices by dialing an extension number.
A popular example of output port usage is to open a door by dialing an extension. The four input ports create an off hook event in Asterisk. The dial plan can be configured to use this function to dial an extension, play a message, send an email, activate another device using the output ports, etc.
Examples of possible input port usage include forwarding a call to specific numbers, and playing a pre-recorded message if the fire/burglar alarm goes off.
For more information, see the various Asterisk application notes on this Web site.