How to set up an android device to function as a phone only
Install this Easy Phone App into your grandmother's cell and place its shortcut on the screen or the desktop of that android device.
And tell her to just swipe that circle with lock off the screen (i.e Unlock  screen) whenever she sees one, and touch on that phone icon in the center! (to activate the Phone app)...

Advantages:

Big font User Interface.

Descriptive Graphics.


Statistics:

Average Rating: 4.4 of 5.

Of 153 people 97  have rated it 5 stars.


Alternative-

Install this One tap dailer.

Features:

It gives a speed dail like facility. Meaning that, it creates a dedicated app icon on screen for each and every contact. That would be good if she has some few fixed people to talk or chat to.

Statistics:

Of 17, 7 rated it as 5 stars.