I'm in New Zealand, but I got one of these for my Mum ( with Alzheimers)
http://www.shoplowvision.com/tek-pal-remote-control.html
She can work it just fine on her TV in her room.
Unfortunately the main TV is on Sky, and only Dad ( with cognitive impairment) can work the Sky TV remote.
However I have still had to confiscate the main TV remote that switches the TV off/on and tape up all the buttons on the TV apart from the On/Off switch, and then Dad uses the Sky remote to switch channels.
Prior to taking the TV remote away, the pair of them were pressing wrong buttons and switching it from Sky back to TV, and couldn't figure out how to switch it back again.
Mum rarely phones now, but will answer and does know how to bring up peoples numbers on the phone directory button. She often doesn't press END at the end of a call, so if people can't get through they know to ring me.
My parents live behind me.
Dad never phones and hardly answers the phone and leaves it for Mum.
He does not know how to bring up the phone directory. Having a big huge phone book there with peoples numbers so he can press the buttons manually is too much for him.
As you can imagine when he wants to ring his brother back in Scotland, my Mum thinks shes doing him a huge favour by bringing up the phone number for him.
Causes arguments
I never explain to either of them how to use any technology now. I just think of the easiest solution if possible.
The only thing I would liem to find is a simpler phone that sounds some kind of alarm if not put back on standby after taking a call, or if Mum fiddles with it, presses the wrong button, so that no one can call as it comes up as an engaged signal.
As far as your Mum remembering you have been to visit or have just called this is the most problematic. Hopefully someone else will be along with suggestions.