There is a VPN server embedded into Smartswitch distribution, which is completely compatible for connection from a Windows PC.
This might be needed for cases when you need to reliably encrypt your communications.
To connect you should use the same login and password that you use to log into web-interface.
After connected users gets dynamic IP address by default.
So to use SIP you should specify in peer's settings that authentication should be done by dynamic IP address and password or by caller ID and password.
Also there is a possibility to specify static internal IP address for a VPN user.
In this case you can specify SIP authentication by static internal IP address.