Ron -- I am the person who set up the PBX on the Mansfield mesh. It was relatively easy to do and you only need to know minimal Linux commands to do the install on the Pi. Do a search for Raspberry Pi and Asterisk and it will return a site that contains the whole image for the PBX and the steps on setup and configuration. Make sure you read all the documentation on the site and also look at the FreePBX documentation too. I believe it took a couple of hours to install and update everything to the latest versions.
Bart -- KF5BYG