This product would probably rank high in my ‘why didn't everyone think of this before’ list. Burnside Telecom (opens in new tab) is all about a mobile phone disguised as a home phone.
It looks just like a normal desktop phone except that it has a protruding antenna and a slightly small screen. The rest is pretty similar to your classic office phone. Corded handset, anthracite colour, and large buttons.
Why would anyone want to install such a line? Firstly, you do not need a phone line for such a thing, which is not only expensive but often underused and might take more than a few weeks for BT to install your line.
Secondly, surprisingly, it might cost you less to use this phone, especially if you call mobiles quite often
Thirdly and most importantly, your granny will feel at ease with it.
I wonder however, if something similar could not be done with T-Mobile Web n Talk. I would not mind a wireless dumb terminal à la OLPC using T-Mobile's SIM.