If your configuration allows you to put the device behind a fiewall, then perhaps you can use a service like dyndns.org in conjunction with one of the routers that supports this service. this would then give your IP phone a URL as opposed to ad IP if your application supports this.
for instance if you go to http://lapaloma.dyndns.org/dev you will see one of my cisco ATAs. I dont need to know the IP address! I use port forwarding on the router , and the Siemens router automaticly regesters IP updates to dyndns.org.