I use the voicemail -> email forwarding option on VFX (great feature), and I noticed a problem where if the voicemail box was full, it would stop recording new messages, and also stop forwarding them to the email address!
However, it seems the guys at WorldxChange can set the system up to email you when the mailbox is close to full.
If you're like me and never dial into the VM system to clear your messages, drop them a line!
Jason