Jamie & I have been talking about reviving this project, and it looks like there's a few more people interested in it now so this might be a good time to kick-off another iteration.
The hang-up of previous designs has been the hook. It's been tough to design something that is both simple enough to work and durable enough for outdoor use. If you look at payphone designs, the hook switch is connected to a lever which goes through the face of the cabinet. I wanted to avoid this to support a flush-mount capability, but I'm learning why the original design was chosen.
So, the next iteration will be more like a traditional payphone hook, and will be designed around an all-in-one cabinet housing the hook switch internally (as well as the electronics).