Good luck with that, because I suspect you're either going to have to roll your own solution or give up on the idea entirely.
Also, I'm not really a big believer in all of this cell phone radiation stuff. For about the past 50 years or so there have been very high energy TV signals being broadcast 24/7, and for even longer than that, there have been high energy radio signals being broadcast 24/7. The output of these signals is measured in Megawatts, and sometimes Gigawatts. Over the air TV signals have a very innate property in that they can permeate almost any structure with ease.
Anyway, anyone still alive today has been subject to bathing in this high energy EM radiation for most of, if not our entire, lives. I suspect that if they've failed to find any kind of link between these signals and negative health effects, that cell phones (which are hundreds of times weaker) are unlikely to be succeeding where everything else has failed.
Which isn't to say that they don't have a sociological and even psychological impact on people. It makes it even easier for us to be contacted no matter where we are, so it's that much harder for us to "get away" and have a little time for ourselves. THAT can have a profound impact on people's behavior. Making us more prone to the various physical results of, for example, stress, which can induce ulcers.
Anyway, getting back to your question... I think your best bet would be to try and get some kind of bluetooth adapter that can plug into an RJ-45 jack. I have no idea if such a beast exists, but you could potentially get an electronics geek to do some rewiring of a VoIP phone.
You might also be able to retrofit some bluetooth adapter into the casing of a regular phone. It would probably have to be corded, since bluetooth doesn't have the same range as cordless phones.
Ultimately, I'd say it's better to just pay the extra money to have a land line. Think of it as a safety precaution, in case all cell service is lost and you need to call an ambulance or something. Because there's no way to escape cell phone radiation... Even if you're not holding them to your head, they're still communicating with the nearest base tower. So just being within the same domicile as a cell phone means you will be exposed to some of its radiation. It's an omni-directional transmission, and it has to be strong enough to reach the nearest tower. Maybe Bob knows about how far apart the towers should be for maximum coverage, but I'd estimate one every 3-4 miles.
If you do come up with something, be sure to post back. I'd find it interesting to hear about on a purely intellectual level.