adamj: Firstly, would they customer link me a Napier number even though I live in Wellington?
I can answer that one for you Adam because we used to do exactly this a few years back. Our office was in Auckland but we also arranged Customer Link numbers in Wgtn and ChCh to be forwarded back to our Auckland number. It gave customers in those calling areas a local number they could call to reach our business.
Therefore, you should have no problem doing that.
adamj: Secondly, I assume I would need the permission of the account holder of the number I temporarily customer link to before they connect it?
No, you shouldn't need any permission. AFAIK, you can forward the incoming calls to wherever you like so long as you pay the appropriate charges, and it is treated as with any other incoming call. However, if the recipient complained, then Telecom would probably cancel the call forwarding. Unlikely to be a problem in your case
