At present if you want to change the outgoing message, you can only do so by reading it out live over the phone, not uploading a previously recorded message.
Ability to upload MP3s (or other formats) of an outgoing voicemail message via the website, rather than having to record them manually.
In a perfect world, you'd be able to upload multiple files, and then assign them to specific lines, time profiles etc. (e.g. one for when office open but busy, another for when office closed) without having to manually replace.