I tried to use macmini as a fax machine. the problem is that Apple used a cheap hardware for modem and the fax reception is shutdown after 1st page gets through!!! I didn't try using PB but I suspect it would do the same since the components are shared. second thing is software. The only commercial solution I know is FaxStf http://www.smithmicro.com/default.tpl?sku=STFMAPEE&group=product_full
If you decide to implement viable fax solution the please share your experiences on this forum.
And yes, fax functionality is built-in to the OS so yes you can just jack the cord in flip couple software settings and be on your way.