Juha: Correct me if I'm wrong, but I think all of this will be sorted out in April next year, with number portability.
I had a look at the Number Portability documents a while back because I thought it might be the solution too. However, it was fairly clear that number portability only applies WITHIN a given free calling area. Sorry, I don't have the URLs to back up this assertion, but it seemed fairly clear when I read the documents.
This is pretty lame I reckon, sort of a "Clayton's Number Portability". It would be interesting to know how this compares with overseas jurisdictions.