Don't need WiFi. You can call via VoIP even without WiFi
If it were for my personal use that would be a solution, but when it comes to [most of] my kids, that wouldn't work. It needs to be foolproof. Which I solved by borrowing a T-mobile SIM from a friend.
Case in point, recently had 160 Shekels added to my Golan bill because my son assumed that unlimited calls to the US, meant unlimited overseas calls, and allowed someone to use his phone for a half hour conversation to a mobile number in the UK.