There are many option for you vpn connection but none are related to VoIP really.
The main reason for this is that you are planning on running over the internet. You have no service level agreement for the connection so you are wasting your time to do QoS on the traffic carried within the VPN.
If this were something like MPLS where you needed to encrypt the data but had a committed bandwidth you would need to worry about doing low latency queuing and the jitter caused by encryption delays.
In your case the jitter on the internet will be so large compared to any you may attempt to control it does little good to even attempt it.
You can just pick your favorite encrytion and key exchange and try it.
