The best and nicest way in which you can hook up your entire household for an all in communications solution has to be a multi handset Digital phone system like on of the Quad Telephone sets over at Telephones Online.
Faced with the expensive propositions in buying and installing a large PBX system or going for a hosted VoIP solution this has to be the most cost effective way to set up so that all your phones in the home can answer the phone and talk to each other.
Many of the telephones online at Quad Handset Cordless Phones have the facility to transfer calls from handset to handset as well as being able to intercom to each other.
More advanced models such as the BT Graphite ( pictured above ) and the BT Edge Cordless Phones have integral answer machines allowing remote access of messages from all handsets.
Of course, dependent upon your needs, number of rooms and the access you wish to grant certain members of your household you may wish to downsize as far as a Twin Cordless phone.