I 100% agree, I have been trying to think of an elegant solution to my rarely used home phone since both wired and cordless phones are crap compared to a modern smartphone. This would be a brilliant answer, bluetooth to my N900 and maybe a wired backup phone for power outages and backup when nobody with a N900 was at home.
This would require an app to do the DTMF for dialing or some sort of integration to the phone dialer app.