I am trying to find a configuration that would do the following:
When extension 1234 receives a call and answers it, Asterisk would immediately place a call to extension 6789, and if 6789 picks up, he would be able to hear the call in progress, but be muted himself.
It would also work fine if when the call came in it started ringing both parties, but if 6789 picked up first it would not be acceptable for the caller to be notified the call had been answered until 1234 picked up, or to hear 6789.
Not saying “No”, but I can’t think of a way to make that happen that way.
You want to use “Chan-Spy”, which I don’t think you can use in this application. If Chan Spy will work, you can use that to connect to the call and be muted or allow “whisper mode”. Add a BLF for the other phone’s “off hook” status and you should be good to go.