I don't see the point in trying to constrain Voxalot numbers to not clash with local 6 digit numbers in Aus. If you do that, then you should do it for the rest of the world and you can almost guarantee that there will be a country somewhere that uses the same 6 digit numbering space.
Far easier to either use the full dialling code when trying to reach a PSTN number or, use a prefix for the PSTN number (or Voxalot number) and have a dial plan entry pick it up, strip the prefix and forward accordingly. It's not too difficult to do
