Is there a way to upload prerecorded audio files to your call-in number, so that you can call in and listen to these recordings when there isn’t actually a meeting online at the moment?
You can use Broadcaster for this: https://www.freeconferencecall.com/support-center/en/en/articles/broadcaster
Every uploaded file will have a reference number, the same as any recording in History & Recordings section. You can then call to your Playback number and listen to an uploaded file.