It's definitely not just a cheaper model. I'm guessing the one she has is a 900mhz 1980's phone. It's ancient.

I didn't get to look at the handset because she was using another phone and I didn't want to pick up while she was talking, but my husband saw it on a previous visit and said that the screen was big enough for one line of text. No idea about the buttons.

Three people before me have looked at the phone and found the manual that you linked, and they gave up on it. No one wants to go with trial-and-error because she has numbers programmed in that she doesn't want to lose.

It would be easier if she just got another phone, but she's not going to do that.