Well ... the question is pretty generic.

But based on these assumptions:
-- no media (bypass media)
-- routing done via XML dialplan

Something along the lines of a quad core machine with 4 gigs of ram would be overkill for 692 calls.

Things to remember:
-- the more cores the better (FS is heavily threaded)
-- the more memory the better
-- 64 bit is way better than 32 bit

SDR