I was looking for a device like this myself, and could not find anything, so I will put it on my wish list for some fun with electronic projects to make someday.
However an alternative for now but not exactly what you want because it still uses the internet however at least you can use the android to answer your landline calls is to get a free google VOIP or other fee based VOIP service like callcentric and set up the softphone app on the phone (softphone not work with google voice, have to use googles software for that) then set your landline to forward all calls to your VOIP phone, if not know how to set a landline to forward calls ask your phone service they will tell you its a few key presses, however your landline will not ring until you deactivate that setting, unless the phone service has option to ring n# of time then forward the call, but probably an extra fee for that.
Anyway that is kind of a work around until someone makes such a thing, but the demand for that is probably low, since the trend in the public is to only have cell service, and not use landlines.
However for me it is a neat idea and want to make it for myself, will probably use scraps from an old useless phone, add in a cheap microcontroller with a bluetooth to talk to the android then write some software to make it all work the way I want it to.
Probably can do it with a PC and PC modem, and route the audio to the bluetooth, but I not want to have a full big box PC on consuming power 24/7 just for that.