I'm not sure that you need this. If your ATA is set up to route all calls to VoXaLot, then all you need to do is dial 500 on your telephone handset, the call will go to VoXaLot. All you need to ensure is that you don't have a VoXaLot dial plan that routes a 3 digit call to one of your VSPs.
Have a look at the very bottom of the tutorial here
http://www.voxalot.com/action/static...lay&itemOID=23 for info on how to set up your VoXaLot dial plan so that the 500 voice mail number is not routed to another VSP.
Matt