I'm glad you are happy with your solution. However, I want to point out that your comparison is factually incorrect. There is no ping-pong, multi-hop internet routing in my solution. Google Voice, by design, can connect to multiple XMPP clients simultaneously. By simply configuring Google Voice on an SPx slot on each phone and ATA, all will be directly rung, and answering the call on one device is no different than answering it on another device. This is as efficient as your LAN solution, and it eliminates the (unlikely) possibility that the phone you're depending on to do the local routing being offline or broken, or that several years down the line, you forgot how you made all those configuration changes.