Mon Aug 19 14:00:19 EDT 2013

The 's' extension

From [1] 

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

From [2] it seems that an extension can be provided in a register

  register => user[:secret[:authuser]]@host[:port][/extension]

[1] http://www.voip-info.org/wiki/view/Asterisk+s+extension
[2] http://www.voip-info.org/wiki/view/Asterisk+SIP+register