I've set up voip instances, and not liked it, but would be willing to do it again for my kid. I'd not be willing to set them up (and be tech support!) for all of his friends' families. That's the value proposition here, for me.
We've got a group of parents around us who'd likewise like to delay their kids' smartphone access for as long as possible - but if a smartphone (or even a dumbphone with no meaningful parental controls) is the only way for kiddo to make calls, then I know some of them will defect. Selling them all on this kit (or something like it) would keep the agreement intact for a while longer.
Several comments in this thread give "Dropbox is just rsync" vibes. I'm curious how many of the commenters suggesting to DIY understand that having small children means essentially no free time to hack on something like that.
And it's mostly tech workers here. I would say most parents are not technically inclined. It would take the average Joe god knows how many hours to set something like this up. Even for a techie, and even if you value your time at only $10/h it would be worth it even if it took only a weekend of hacking something together, and you get something that was built specifically to your use-case.
It really isn't. I got my login for dropbox, installed it on some machines, and it was just click upload or download from there. Crwating and using folders was much like on my desktop.
For rsync, a person would have to study it to learn it. They might want to look for potential gotchas in how they configure it, too. The experts at Dropbox already did all that for us, though.
Your residential internet provider will probably already sell you VoIP that you can plug a real phone into.
Put that old hamburger phone to good use.