Why answer the question if you are "not familiar" with it? I am getting this a lot lately to a number of numbers, and when I call the fax lines, they are beeping and ready to get the fax. SO, knowing what "Unknown" is would help me know how to resolve this issue.

I have had the same issue and while working with support they told me the receiver is hanging up. I spoke with the receiver and they are using a stand alone fax machine that has no issues receiving from others. I have also tested sending from another online fax account and it works every time. Something is up with RingCentral's faxing service. This has been on going since March 2022, starting then the result is either unknown or busy. If you get this resolved please come back here with the fix!

I'm having the same issue and need confirmation that the faxes are going through. These faxes have a 24 hour response time and it is important to know if the faxes are successful or not.