You really can't get around the fact that the solutions which have been produced thus far are not ideal for plunking your cell phone in a dock at home to have it also ring on a handset in another room. I had a friend which tried one (Panasonic? Siemens?) and he commented the sound quality wasn't up to par considering what he paid. I'm not even certain they're still being made, but I'll check again. I don't recall it being especially cheap either. Keep in mind this means you would only hear the cell phone ring once you left home- anyone needing phone service in your residence after you've left w/ the cell would be w/o service unless they had their own wireless phone/line per se.
Have you though about even keeping a basic no frills land line alive, such as the cheapest ATT service (~$15 + tax/month)?
I'm not sure if something like Google Voice with all its fancy forwarding options would help you per se since you emphasize how important the single line/service is to you. A bluetooth headset wouldn't give the proper range IME, so that solution's out as well ;(.
I need a cell phone. I want to keep my current phone number.
I have a land line with extension phones. I like that. I cannot afford two services.
Is there a service that will allow me to have phones docked at home and a cell to carry with me?
I don?t want to buy several cell phones and leave them all over the house, nor do I want to buy one and carry it everywhere.