You can actually get an existing phone number converted to a VOIP line. I doubt it's possible with Skype, but it is possible with some other providers.
It's called NakedDSL. Depending on your area/existing phone line, the broadband provider basically just uses your existing copper lines to provide broadband to your residence and converts your phone no. over to a voip service. iinet is one provider that does it. You can find out more here...
http://www.gonaked.com.au/what_is_naked.php
Cheers,
Anthony.