I've used m0n0wall on alix and soekris boards, and pfsense on soekris. I prefer the soekris hardware, but both are nice and compact, low power. If you get tired of asterisk you can install the freeswitch package on pfsense, like I did. The freeswitch package is well-supported and maturing rapidly. (The freeswitch software itself appears to be rock-solid. When I say the package is maturing rapidly, I'm referring to the GUI, which is heroicly maintained by mcrane.)
db