First of all, JustChat Requests from you to enter your phone number, and then send a verification code to the entered number, in order avoid any security issues.
also, requesting a permission to access your contact and append the contacts from your mobile that are using JustChat to Your Friends list.
JustChat also, saving all your messages and friends, so if you changed your mobile phone or forget your friend phone number, then you can easily restore all your messages and your previous friends.