If you control both routers in your network you can purchase routers that can be configured to support a VPN between the two routers. Some routers can also be flashed with 3d party router software that contains VPN software to allow you to setup a VPN between the two routers. Examples of the 3d party router software would include DD-Wrt and Tomato.
One voip service provider, CallWithUs, offers a VPN link for a yearly fee using OpenVPN client software. To use this approach, I believe you would need to install the client software on your pc and using internet connection sharing to attach your SPA3102 to the pc.
