A few months back I came across a small integrated device costing around GBP 100 that was a complete computer with RAM, hard drive, USB and network connections, pluggable with a pair of FXS/FXO modules. Just plug in your FXS/FXO modules, install Linux and Asterisk, and go. Darned if I can find it now! Link anyone?
If you find it please post it here. I would like to get one as well.