Add a remote extension to my existing FreePBX system

I have FreePBX setup on a Raspberry Pi, and it's been working great for about 2 years. Now I need to add a remote extension, and I'm looking for someone who knows the pros and cons of the different methods (SIP, IAX2, VPN, etc.) who can let me know the most secure method, and to set it up my FreePBX server with the appropriate settings, then to provide instructions for me to setup the remote phone myself (remote phone is an Android smartphone which will have a fixed IP address on a home network behind a NAT). My budget is $75 or less.