Digium, Inc.
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Chapter 4: FXS and FXO Explained
Using Your Analog 410 Series Card
Connect the outside line to an FXO port on your Asterisk server to receive
voltage from the outside lines.
Connect the phones to FXS ports on your Asterisk server. When the FXO
module in your Asterisk Server receives the voltage, it will then generate
voltage using the FXS module and send it to your analog phone.