Best VPN option for Asterisk Voip

Which would be a recommended VPN option for a Asterisk Voip service over the VPN between several branches in an enterprise

Someone can help?

I have a setup working with a Goip1 (gsm gateway) and a router with OpenWRT .

When someone from the office is going to a foreign country for, imagine 2 weeks they take this setup with them. In the hotel they plug the router to the power and internet. Automatically the router makes a vpn tunnel (OpenVPN) with our main office and register the gsm gateway in our asterisk. In every country they buy 2 prepaid cell phone card, 1 goes to the GOIP1 and the other to the cell phone.
This way we can communicate in a very very cheap way and sometimes we don’t pay the calls between those 2 cards.

So the answer to your question is OpenVPN.
This setup is working for almost 2 years.