Frankly, I have no idea what VIA does
All I am saying is that I assumed that EXT IP helps STUN to communicate WAN IP address, and I found out that STUN doesn't need any help. So, I removed EXT IP and it had no bad effect. In fact, in the end I was able to direct connect (both connection and voice) by just forwarding SIP and RTP ports on the router.
I played with direct dialing a month ago, but now all my adapters are serving friends and family and are registered to VSP. And I have several VoIP devices at home - so I am very nervous to forward RTP ports to one of them. So, I don't have equipment to test at this time...
Ext IP is unrelated to STUN. STUN transmits the WAN IP address to the ATA from an external server (Stun Server). Ext IP provides the WAN address directly using the IP value that you supply without the use of STUN.