How to transfer a call to a trunk if the extension is not available?

Question

Asterisk

The scheme is aterisk plus one outgoing sip trunk that receives all outgoing calls. Adding several hundred extensions, you need to make sure that a call from one extension number to another extension number is transferred to a single trunk, provided that the called subscriber is not registered at the station, i.e. is in the unreachable state.

Answer 1

Alternatively, the asterisk has a DEVICE_STATE function.
In your case, you can implement the scheme, for example:

if (${DEVICE_STATE(${EXTEN})} = UNAVAILABLE) {
Dial(SIP/[email protected]);
};