I saw this phone online which is probably as close to a home phone as you can get. It is only able to call 3 people but would be easy for someone like your Mil to use. I don't know if it is available in NZ or price, so may not be very helpful.
https://www.doro.com/en-gb/shop/mobile-d...doro-780x/
https://www.doro.com/en-gb/shop/mobile-d...doro-780x/