My grandmother has an Ipad and an IPhone. We sometimes video call her on here phone using whatsapp. But this works around 50% of the time correctly and the other times we just call using a regular land line phone. She uses the Ipad only as a image viewer. We showed her multiple times how to google something but that is just to difficult for her. But she understands how to watch the photo's we put on the device, and she really likes to watch the old photo's of all our vacations together. So my recommendation would be use something that your grandmother is already comfortable with and start thinking from there. Introducing something new can be hard for her even if it is simpeler.