Anyone used Tox? What is your experience? Will it be easy for people who don't have technical know-how?
You haven't given us any information about what they have access to: Phones? Laptops? Desktops?
You haven't given us any information about what software they run: Android? MacOS? Linux? Windows?
You haven't given us any information about what you've considered and rejected: WhatsApp? Zoom? Facebook Rooms? Messenger?
What are your constraints?
I don't have an answer, but information like this seems an absolute minimum to let people help you.