Despite Fulvios statement that he is intending to write one, i do not think that it necesarily should be included.
Zeroshell is an excellent router/fireewall/vpn box and i cannot see any relavance to it being an asterisk box aswell.
Indeed i would certainly run asterisk on a seperate machine or virtual machine. There is no benefit of running asterisk on your zeroshell edge router.
My feeling is that concentarting on routing/authentication functions would be better i.e.
More configurable radius
Why do you want asterisk on it?