While the geeks came up with tons of great suggestions, like an Asterisk VoIP system, Google Voice came to my mind as a perfect source to meet all the needs and requirements.  Here is why:

• Inbound calls at first could be answered on any phone and if it wasn’t for that person you could press * (star) and it will ring the other phones.  Let them pick up
• As Google Voice gets calls, you could then start routing specific calls to only certain phones through the settings for that contact entry
• With the above, Google Voice becomes a household phonebook and directory.  Can’t find your daughter’s friends phone number, look in Google Voice.  That person has a directory entry with all the info you need.
• Want to route all calls to a temporary number while you are away, like on vacation, to save cell minutes and costs?  Easy as can be.  You can create temp entries even on the road with your pin number.  This help document describes it all.
• You can have the system screen all callers, unknown callers or none.  Your choice and your way to know if it is for you
• Built in conference calling.  Yes Google Voice can do conference calling on inbound calls to the voice number.  Your phone must support call waiting for this to work, which almost all do.
• Everyone can call out as that number to make it appear as a home phone, protecting your cell numbers.
• Everyone can use the native Google Voice app on Android and Blackberry, (as we wait for the iPhone approval still), the computer and other browsers to check voicemail and answer texts.

There are many more items that make it an easy sell.  I think this is a wave (no pun) of the future of calling services as what device you use does not matter.