Voicemail greeting files

I have an MP3 file to be used as the greeting on the reception voice mail.

I know I need to convert the format so I searched the web and came across a posting which gave this command:

       sox input.wav -r 8000 -c 1 -s foo-out.wav resample -ql

My sox complains that resample is depricated so I replaced with rate -m

The resulting file plays fine but when I replace the unavailble.wav the voice mail for that extension stops working. I have chmod and chown so permissions are as original unavailble.wav.

On the CLI I see no errors.

Removing the files and we are back to the default message.

Replacing the original unavailble.wav and still works fine.

The only possibility that I can think of is that the wav file is not of the correct format.