Asterisk as a "Virtual Office" / "Virtual PBX

What would be the best way to go about using Asterisk to duplicate the functionalities of what others market as “Virtual Offices”…such as these? What would the strengths of the Asterisk solution be over these “hosted solutions”…what would the weaknesses of the Asterisk solution be?

the first one is…
http://www.virtualpbx.com/services/comparison.asp

and the second one is…
http://www.tollfreelive.com/800-numbers/800-numbers.aspx?p=vo

CHEERS!

D