I am not completely clear on the method described here.
In my case, we have two separate LANs in a single office suite. Each LAN has its own ISP and router, DHCP server, etc. We now need the 2 LANs to be connected, in order to share an IP phone system, but need to maintain our separate LANs, ISPs, routers and, in general, keep the 2 LANs isolated from each other, except for the ports required for the phone system.
LAN 1 is 192.168.1.x
LAN 2 is 192.168.0.x
Does it make any difference which LAN is connected to the LAN port on "Router C" and which is connected to the WAN port?
Also, suggested routers would be helpful, as I do not think all routers let you choose between NAT routing and classic routing.