When you use the voicemail system and forward a voicemail from one box to another it leaves the message you forward in the original mailbox and places a copy into the new mailbox. The ARI on the other hand MOVES the file from one box to the other so that the person who forwarded the message no longer has a copy. This becomes important if you wanted to say forward the message to more then one person, on the voicemail system you can but in the ARI you are out of luck and also just lost a voicemail that might be important that you keep.
If and when the voicemail system and web interface is redone and implemented in the style of a e-mail system as most new ones are it would then also match that operation.
Quickly looking it looks like if you add an additional parameter to the moveVoicemailData routine that says yes to move or no to only copy and then use that parameter to control the unlink operation it would then solve the problem. you'd only need to update the two locations where it is called from. But I'm no expert in PHP or how it works with files...
