alt Hacker NewsI "made" something like this 7-8 years ago for our first kid. When I say "made" I mean I bought a "fixed wireless terminal" for $40 on eBay, a classic landline phone for $30, and a cell phone subscription for kids ($5/month). Then I connected the parts, and voila, we had a landline for kids.
Obvious benefits include low cost, full interop with all other phones, and having the kids learn our phone numbers by heart after punching them many times.
I made something using a VOIP phone and voip.ms. What provider has a $5/mo kids cell subscription!?
voip.ms is about $1.30/mo with the cost of the phone number (DID) and "minutes".
I have some more breadcrumbs on this thread if you are interested in details on my setup too.