A VPN is a generic term used to describe encrypting a traffic flow over the internet. VPN functionality can exist in software on a PC or software on a broadband router/firewall.
Most typical users of voice over the internet seem to use programs that run on a PC (ie Skype). AnchorFree's Hotspot Shield and the pay services work great for PC-based voice programs only.
VPN software you install on the PC does not help with services that use a stand alone adapter (Vonage, Packet 8). However, there are plenty of ways to unlock those too