The application gateway 1000/2000 product was spun off to Citrix before being finally discontinued. Even if you can find a copy of the installation media (it's a few separate pieces of software that need to be installed on the server), you would need licensing. They licensed the product by server, by feature, and additionally by the individual phones you want connected to use the features.
There's a knockoff product from Russia called SoftDrom RFBServer, but it's quite old and not fully polished. If you can find a copy online and are comfortable with running unvetted software, it could be something to play with.