I did a little experimenting using
@jerrm definitions, I started with:
greet.wav.bak 14444 bytes (uncompressed)
greet.WAV.bak 1490 bytes (compressed)
I copied these in various combinations and tested whether I got a voicemail greeting rather than a hangup when calling the unanswered extension:
WORKS: greet.wav bak -> greet.wav and greet.WAV.bak -> greet.WAV
WORKS: greet.wav bak -> greet.wav and greet.wav.bak -> greet.WAV
WORKS: greet.WAV bak -> greet.wav and greet.WAV.bak -> greet.WAV
WORKS: greet.WAV.bak -> greet.WAV and NO greet.wav
WORKS: greet.wav.bak -> greet.wav and NO greet.WAV
FAILS: greet.WAV.bak -> greet.wav and NO greet.WAV
FAILS: greet.wav.bak -> greet.WAV and NO greet.wav
So it looks like any combination works as long as you have two files even if they are mismatched and incorrectly named. But, if you only have a single file, it has to be named correctly or the answer fails. This still doesn't explain why the Asterisk 13 files are improperly named unless that is happening as part of the restore process.
@kenn10: Can you verify the names of the greet files on your Asterisk 13 platform before the restore? If they are .WAV files and being imported and renamed as .wav files, that would obviously be another bug even though we have solved it with restore-fix code above.