We switched from a Windows-based 3CX to a Debian instance, and migrated the recordings over.
The recording path is set by default here:
However, you need to be careful and use the phonesystem user as the owner of the recording folder, and subsequent folders inside, otherwise recordings will stop being saved.
This can happen when you restore this folder from Windows, using the root user.
In that case, doing a ls -l will show you something like this:
Get there via SSH and apply a recursive permission to the root folder:
chown -R phonesystem:phonesystem *
No service restart is required, the 3CX will start recording to the extension folder right away.