I’m still having this issue with the latest version of FreePBX 16. I think I found the bug though. If the Ring Group is set to ringall, and the FMFM is set to ringallv2, you can answer from the external number without issues, but you can’t answer from the desk phone. It will say the call is not available anymore and hang up everyone.
I’m on FreePBX 16.0.40.7