OpenVPN is based on the Secured Socket Layer (SSL) which sits on top of the IP stack. With OpenVPN you can select variousauthentication methods and encryption algorithms. It can also provide data stream compression.
In most of configuration, we will have a main site which will act as the VPN Server and either fixed remote site with interconnected LAN and/or roaming mobile users.
We assume that LAN interconnection is done at IP level while Roaming users are connected via a bridged Ethernet.
The general configuration could be as described bellow.
SIP signalling and voice media take independent IP routes. Futhermore the up and down streams are independently routed. 
When you run a phone service over the public internet there is not much that you can do to optimise the service. The quality of the phone service will be dictated by the IP packet loss, the IP latency and the time require to dial.
The roaming use case represents the need of a remote user who needs to connect to a main LAN in a transparent manner. Ideally, the remote and main LAN users will see no difference between roaming and locally connected users. The IP addressing plan will be the same, non IP protocol (e.g. printer and share discovery, Novell directory, ...) and IP multicast will work. 
The Lan Interconnection use case covers the need of small office remote users who need to be permanently connected to a main LAN. At fridu.org we think that remote offices must stay operational even if the DSL link is down. To achieve this independence, we propose an IP interconnection and not an Ethernet bridge mode as covered in the roaming user use case. 
