Just for the record in case anyone comes across this thread when searching for info on phones, I recently bought a Doro PhoneEasy 331ph as suggested by
@nitram.
It does the job, but I was disappointed that various controls are a bit too easy for a person with dementia to fiddle with (if they're that way inclined, of course; my dad certainly is).
In particular, the volume control is flimsy and would be easy to move even without meaning to. The top volume level is not all that loud, so I took the cover off the phone and snipped the two wires to the volume control potentiometer. You can still spin the dial but it now has no effect, and the volume is permanently on maximum.
Again with the cover off, the ringer volume control and various other switches were Dad-proofed by removing the little plastic toggles that fit over the switches themselves. You can still operate the switches, but only by using a paperclip or a screwdriver rather than fingers.
In conclusion, this phone is on the cheap-and-cheerful side. it will work perfectly well for most people but it would have been nice to see more thought given to dementia-proofing it.