Need help with translating a single IP


I hope what I want is possible...

Consider this: I have two networks, Site1 ( and Site2 ( They are connected with a site-to-site IPSEC tunnel.

There is an IP in Site1, that I want to "become" in Site2.

I thought to realize this with Full NAT, but that does squat in this case.


Longer explanation:

I have a VoIP phone in Site1 which I want to connect to the Phone-Server in Site2. While this does work when I let the IP remain (Phone-Server registers the IP as that one). However, for other reasons, I need the Phone-Server to register the Phone as if the phone were locally in Site2, having address. So I want to change the IP of the packet as it travels towards the phone-server.

Ideas please?