Wayne,
I used RingCentral which also has eXtremeFax. I used it exclusively for Internet faxing and found it way too expensive. Of course, now I fax for free. I wasn't aware that RingCentral had expanded its offering to be more of a OneBox-type program. In my experience OneBox has many more offerings and functionality than RingCentral. Plus, it couples with UpDox and is much cheaper that way.
If you are planning on using it for the weekends, etc., please don't do ANYTHING without talking to Adam as he has been using OneBox not as a receptionist (there are three ways to use it) but as his answering service. He loves it and knows exactly how to use it and has spoken directly with Andrew Barbash who, of course, knows OneBox and UpDox better than anyone.