format
The format setting selects audio file format(s) to use when storing voice mail messages. The value is a string defining the audio format(s) of the message file. The default format string is wav49|gsm|wav, meaning that Asterisk will save the voicemail message in all three supported formats. When emailing the attachment, however, it will send only the first of the formats defined here. When playing back (as with all file playback) Asterisk will attempt to use the optimum format based on the codec used for the current channel, in order to provide the best sound quality and to reduce transcoding processing time.

* wav49: In this format, the file size will be small, the quality good, and it's a good choice for sending voicemail messages in email. The file will have a .WAV extension, which all Windows users should have no problems with, and users on other platforms should also be able to easily play these sound files.
* gsm: Voicemail saved in this format will have about the same file size and same audio quality as wav49. It may be less well supported by client operating systems if sent to users in email, however.
* wav: This is an uncompressed sound format with a .wav extension, so the file size is very large. Sound quality will be great, but you probably don't want to email it, and you must have adequate disk space.
* g723sf: The sample voicemail.conf file has this option commented out. If you try to activate it, you will find that it doesn't work.