Phone numbers are too public. The reason why they're used by messaging apps is that they are a goldmine to have. They actually make it harder to chat (ever tried using Whatsapp/Signal on a PC? Yes, you'll need to have it installed on your phone first (and have given over your contacts))

That's why I chose to set (masked) emails as the primary id on groupsapp.online and even these can't be seen publicly unless you share a "group". Others will just see XXXX@gmail.com