With regular PSTN there is a feature sold here as “Busy Call Return” that will continue to try to ring someone if you called them and the line was busy. Once they’re line is free it will ring you.
I have not found this feature in FPBX and I’m wondering if it is available under a different name, or not available?
I think what you are looking for is called Campon, but not sure if it works exactly as you’ve described.
That is what I am looking for 
Unfortunately, it is internally only from what I can tell. I am looking for something that would work outside fpbx when there is a busy signal. I saw a user UvoiP had created something like this, but not sure how to get in touch with him (also the module may not be maintained) so I was hoping the official modules would have it.
Unfortunately that is not possible, only the “carrier” of that number is able to know that privileged information.
BUT, you could could build a custom context or agi script that continually called that number until it answered and then transfer it to the original calling extension after letting the called party know what the hell is going on (this would not be trivial 
)
Thanks for that info…it’s good to know it can’t already be done. will look into the scripting method you mention. Although my programming skills are quite rusty, maybe I will have some luck