Looks like you will need a gatekeeper.A gatekeeper knows the I.P. address of each phone, and will let you dial a phone number instead of dialing the I.P.
Seems a lot of trouble for just two phones.
You could get a free software on the internet that will run into any old P.C. and act as a gatekeeper.
Follow this link: http://www.gnugk.org
and do not get scared by all the mumble jumbo on the site.
There is a file somewhere that all what you need to do is load into your windows P.C. and runs on the background.
Hope that that helps.