[Asterisk-Users] How can I know if a user is busy or not connected?

[Dan]

Hi,

I have defined several extensions.
For some of them, the phone can be disconnected for a period of time (mobile
users).
When a call is initiated to that extension, if the user is not connected at
that moment in time the caller see this as a busy extension.
How can I detect from the extensions.conf file if a user is busy or it is
not connected?
I want to execute different applications in those two situations.

Thanks,
Dan