Asterisk is an open-source software-based telephone system. Those overpriced little boxes called "telephone systems" that give you little control over how your phone system works, are giving way to PCs and routers running Asterisk and similar open-source PBXes. Asterisk allows unprecedented flexibility while still providing the full-featured experience that companies need. If you're still using one of those old closed-box telephone systems, give Asterisk a try.