A post made by @BrendanMays regarding a phone that whilst using the mobile network looks and acts like an old style phone that may be of interest and help to you:
The monthly cost is $55 and the set up fee is $99. It is a leased service so your mother would never own the phone but as it is nothing more than a base station and a cordless phone handset (with largish digits on it) it should be simple for her to use.
