What audio format do I need to upload for my voicemail or virtual receptionist greeting?

The format should be MP3 with PCM, 16 bit, 8 KHz, mono encoding. 

We recommend using the free Audacity software to create your MP3, and configure it with the above settings. Alternatively, you can use some online tools such an Online Audio Converter.

Note that higher quality MP3s will NOT work since telephony use g711 encoding, which results in a substantial bandwidth cut. These MP3s would effectively just occupy more space, and most likely will not even be recognized by the decoder.