Thank you for posting your question in the Support Community. You may setup a voice user (connected to FXS) as a hotline:
voice user 1001
I don't know of a way to apply a delay/timer to behave as a warm line. Perhaps someone else will have another idea.
I do not know of a way to get the FXS port to do that. I am not even sure that you can get a notification via SNMP that an FXS port has been offhook for an extended period of time. I believe you can get basic onhook/offhook status, but nothing that specific. Perhaps you can have a script check the FXS status periodically and compare the status versus FXS lines that are part of an active call.
That is the best I have.