Its been a while since i played with ipkg but with the help of
you should be able to find all them you need which is basically apache php mysql and bash if busybox is too restricted to run the ‘install_amp’ script, if you are using 13 then the ‘install’ script is php and the needed requirements might be quite significant,
If you can’t find them for your hardware you could always cross-compile on a more powerful machine
freepbx is likely too demanding to run the gui and successfully process calls at the same time on such hardware though. Let us know how it turns out