The "s" extension is used when there is no known called number in the context used.

The "s" extension is used when starting a call. It is also used when defining a macro.

Incoming calls are always placed in a context in the dialplan, either one you specify in the channel configuration file, or the default context. If no other match exist for the call within the context, the s extension is activated.

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• Asterisk standard extensions: Overview of standard configurations
• Asterisk config extensions.conf: The dial plan
• Asterisk channels