Did this exact thing for my four and six-year-old kids. I used an Ooma Telo Air[1] (Free + ~$6/mo in taxes and fees) and an old vTech landline phone. It's been highly successful in our house. The kids have (monitored) independence to call grandparents and aunts/uncles. Watching them translate written down 10-digit phone numbers into button presses is fun too.

1. https://www.ooma.com/home-phone-service/basic/