Hello,
The phone shouldn't be sending the IP address, it should be using the hostname you specified.
I actually did an article, albeit about a NAT issue, on the Cisco 7940 which goes into detail about the SIP packets going to and from the server [1]. As you can see from this article, the phone is sending the hostname in the To field.
Therefore, this could either be a dodgy firmware or some config option you have set wrong.
Can you please give us the following;
* FS SIP trace logs showing the Cisco phone during registration and/or call state
* Cisco phone config (this can be extracted using the telnet server if you are not using TFTP)
Hope this helps
Cal