You must provide a voice message that is in Waveform Audio File Format (wav extension) file format and is smaller than 10MB. If you have not uploaded a file, the Voicemail button is not available.You can only use Voicemail Drop if you are using CCXML for outbound calls.
You cannot leave a voice message when more than two parties are connected.
If you are recording the call, the recording does not currently include the voice message.
This feature is not supported in IE8 or 9.