I have run VOIP using the free X-Ten SIP protocol soft phone on a laptop, inside a PPTP VPN, which ran inside an 802.11g 128-bit WEP encrypted Wi-Fi connection. The PPTP VPN continued across the Internet to an Asterisk voice PBX at an other location, serviced by a different ISP. The audio quality was fine.
Running the SIP protocol inside a VPN removes any QoS benefits (if there are any) that the SIP protocol might have running naked across the Internet. However, by running SIP inside a VPN, you eliminate all the firewall and network address translation problems you would otherwise have. Running the SIP protocol naked across the Internet requires a a number of ports to be opened up through the firewall. It is quite messy in that regard. When you run it inside a VPN, you are home free, with no firewall modifications required.
The only VOIP protocol that I run straight in through my firewall, without tunneling it through a VPN, is the IAX2 (Inter Asterisk eXchange) telephone trunking protocol, but it uses just a single UDP port (4569) and authenticates using certificates.
