How can I automatically save VMail greetings in ulaw format?

Hi, I use ulaw as my preferred codec, and I’d like to use ulaw end-to-end.

To prevent transcoding from occuring when the voicemail unavailable greeting is played to callers, I’d like Asterisk to play the ulaw version of the greeting.

My question is, since the greeting is created by the user, and can be re-recorded at any time, what’s the best way to do that? I figure one approach is to have a process that calls sox after the voicemail unavailable message is recorded, and sox would convert the file to ulaw format. However, I’m unclear how to automatically call sox, and what the sox command would be.

Thanks in advance for your words of wisdom.