There are several potential benefits to setting up a VPN to your Asterisk server. All traffic is encrypted and you don’t need to open lots of ports in the firewall. Also there are no issues with SIP and NAT as traffic is routed over the VPN tunnel.
This is a pretty advanced setup but here is a walkthrough for setting up a SysAdminMan VPS as an OpenVPN server and then connecting to it with a TP-LINK router running OpenWRT.
First you need to flash OpenWRT on to the router. This replaces the original firmware. Here are some instructions for this TP-Link router - http://wiki.openwrt.org/toh/tp-link/tl-wr1043nd?s. I got version 18 of the router and flashed Backfire 10.03.1-rc6 version of OpenWRT.
I’ve written quite a few guides for setting up FreePBX and A2Billing, but they can look a little daunting. So I though I’d have a go at some video tutorials.
The first one is for setting up an extension in FreePBX.