If you dial your own phone number at home to access the Voice Portal, this will not work if you have turned off the option to "Send Busy Calls to Voice Mail" which is normally on by default. This option is under Messaging - Voice Management on the myvfx website.
My wife kept complaining she couldn't access Voice Mail from home using our own phone number. (She finds that easier to remember than the 082210 or 0800 141414 numbers - which do work).
We prefer people calling us to get a busy tone rather than being switched to Voice Mail if our line is busy.
The VFX Help Desk was not aware of this issue when I called them. I've added this to this Forum in case you have the same problem.