We need a phone that works off of WiFi.?

My wife's aunt is in her nineties and lives in a nursing home that has WiFi. She has been using a flip phone to make the rather limited calls that she makes. Lately, she has been having more trouble doing that. I Think that is is mostly about pressing the buttons. The ideal phone that we would like to get for her would function like the phones that she used forty years ago and work of the nursing home WiFi. We looked at Magic Jack but is seems to require a network plug to work. Right now, it looks like Ooma might be her best bet. She does have a computer in her room that she uses exclusively for email. Any new technology requires very specific written instructions, even if it seems intuitive to my wife and me.