Because VoIP transmits digitized voice as a stream of data, the VoIP VPN solution accomplishes voice encryption quite simply, applying standard data-encryption mechanisms inherently available in the collection of protocols used to implement a VPN.
There is no encryption in former case, but traffic overhead is significantly lower in comparison with IPsec tunnel.
The advantage of OpenVPN tunneling is that it can run on a dynamic IP and may provide up to 512 bits SSL encryption.
A VPN is also a workaround to avoid a firewall issue when configuring remote VoIP clients.
A worker at a remote location could therefore appear virtually to be at the company's main office, with full internal access to telephone and network.