I understand that call forwarding currently works only if you don't have an ATA registered with Voxalot AND the forward is dependant on the calling party, rather than the called 'number'. There is also a catchall that doesn't care whom called you from where; but there is nothing that ties in to a particular incoming registered provider's DID to forward based on that. Perhaps you need to go back to the VSP and see if they can do the forward as you require rather than passing it onto Voxalot.
Here is a link to the "Call Forwarding" tutorial:
http://www.voxalot.com/action/static...lay&itemOID=32