we have a UC540 at the main office, and a router 881 in a remote office. We perform a Site to Site VPN between the office and remote site in which physical phone installed.
Is this scenario possible? or need for physical phones at the remote site to register on my uc540 central office?
another scenario would be to use two ROUTERS 881 both the central office to the remote site and perform a site to site VPN, the uc540 would be found behind the router 881.
At the remote site you only need an FXO gateway.
Again, get to work with a good consultant or Cisco partner, otherwise you will never end having questions but no ready answer
I am not sure about going forward with the UC-320 Option, unless something has changed with it I do not believe it will achieve what you want as it is not an IOS based device and may nto integrate with a UC correctly without some heavy CLI work around's on the UC-500 side.
Also Further to what Paolo has advised, you can also consider the Cisco 6900 series phones if you want a cost alternative product, which generally perform better than the SPA series for voice quality (IMHO).
Yes, VPN can transport traffic for any application.
Thank you for the nice rating and good luck!