Interesting hack, converting a popoplug to use for other purposes. I have a couple of these things kicking around (replaced with other systems). SO this maybe a good way to repurpose them.
Another good hack here to make one a PBX.
http://nerdvittles.com/?p=10560
Nice 30 minute video on getting started here
https://www.youtube.com/watch?v=SheuimfB6kQ&list=PL2GALWZIiXOCgRfWSQpIFc4QfINjOvsYr&index=3
Oh and notes