It looks like Orcon have adopted a fairly standard practice for ISPs - any customer wishing to use a guaranteed fixed ip address must now pay for it - see http://www.orcon.net.nz/about/article/static_ip_addresses.

How volatile is an assigned IP address?  My understanding is that for your IP address to change your router would have to loose its connection to Orcon.  This means that as long as your router remains up and working your IP address wont change.  Genius VOIP users are unlikely ever to voluntarily power cycle/reboot/reset etc. because to do so takes out their phone service.  If this is correct then IP address changes should be rare.