Though the distro is free "as in beer" its license is not. The distro is built with stability and supportability in mind. That said we do not allow "respins" of the distro.
Please see: https://www.freepbx.org/about-us/freepbx-distro/
You are free to build your own base distro and use FreePBX as many have. FreePBX is free as in "freedom" and free as in "beer"
Feel free to see the manual install instructions on our wiki.
http://wiki.freepbx.org/pages/viewpage.action?pageId=1048598