While trying to assign permissions to users in TSAdmin, Telescope would throw a prompt asking for the user's phone number if the user profile already didn't have it. This was especially an issue in versions before 2020.1 because there was no way for a user to provide their phone number at registration which could have prevented this issue. Users had to later visit the relevant Settings section to update their phone number and then only could the Administrator assign the permissions normally.
This issue was fixed in Telescope Standard 2020.1 by making it optional to provide their phone number on the self-registration form while registering. It is recommended to upgrade your Telescope version to this or later to fix this issue. If your Telescope installation has customizations or you're an OnDemand customer, please contact your Account Manager or support for help with upgrading.
Any users registering on a Telescope portal running 2020.1 or later will have an option to provide their phone number at registration which will prevent this issue.